University  of  California  •  Berkeley 


THE  LIBRARY 

OF 

THE  UNIVERSITY 

OF  CALIFORNIA 


PRESENTED  BY 

PROF.  CHARLES  A.  KOFOID  AND 

MRS.  PRUDENCE  W.  KOFOID 


C0>JVERSAT10NS 

ON 

NATURAL  PHILOSOPHY, 

IN   WHICH 

THE  ELEMENTS 

OF  THAT  SCIENCE  ARE  FAMILIARLY  EXPLAINED, 

AND    ABAPTKD    TO 

THE  COMPREHENSION  OF  YOUNG  PUPILS, 

Illustrated  with  Plates. 


BY  THE  AUTHOR  OP  CONVEHSATIONS  ON    CHEMISTRY,   AND  CONVER- 
SATIONS ON  POLITICAL  ECOSrOMY. 


PHILADELPHIA: 

PUBLISHED  AND  SOLD  BY  J.  Y.  HUMFHRET8« 


Thomas  Town,  Printer, 

1820. 


^1 


\^c^L 


3 


PREFACE. 


It  is  with  increased  diffidence  that  the  au- 
thor offers  this  little  work  to  the  public  The 
encouraging  reception  which  the  Conversati- 
ons on  Chemistry,  and  Political  Economy  have 
met  with,  has  induced  her  to  venture  on  pub- 
lishing a  short  course  on  Natural  Philosophy ; 
but  not  without  the  greatest  apprehensions  for 
its  success.  Her  ignorance  of  mathematics, 
and  the  imperfect  knov/lcdge  of  natural  phi- 
losophy which  that  disadvantage  necessarily 
implies,  renders  her  fully  sensible  of  her  in- 
competency to  treat  the  subject  in  any  other 
way  than  in  the  form  of  a  familiar  explana- 
tion of  the  first  elements,  for  the  use  of  very 
young  pupils.  It  is  the  hope  of  having  done 
this  in  a  manner  that  may  engage  their  at- 
tention, which  encourages  her  to  offer  them 
these  additional  lessons. 


PREFACE. 

They  are  intended,  in  a  course  of  ele- 
mentary science,  to  precede  the  Conver- 
sations on  Chemistry;  and  were  actually 
written  previous  to  either  of  her  former 
publications. 


CONTENTS. 


CONVERSATION  I. 

Pag-e 

O^    GENERAL  PROPERTIES    OF  BODIES.  13 

Introhuctiox. — General  Properties  of  Bodies. — ^Impene- 
trability.— Extension. — Figure. — Divisibility.— Inertia. — 
Attraction. — Attraction  of  Cohesion. — Density. — Rarity, 
— Heat. — Attraction  of  Gravitation. 


CONVERSATION  U. 

ON    THE    ATTRACTION    OF    GRAVITY. 

Attraclion  of  Gravitation,  continued. — Of  Weight. — Of 
the  Fall  of  Bodies.  Of  the  resistance  of  the  Air. — Of 
the  Ascent  of  Lig-ht  Bodies. 


CONVERSATION  HI. 

ON    THE    LAWS    OF    MOTION^  43 

Of  Motion. — Of  th^  Inertia  ofBodies^ — Of  Force  to  Pfo- 
diice  Motion. —  Direction  of  Motion. — Velocity,  absolute 
and  relative. — Uniform  Motion^^ — Retarded  Motion*— Ac*- 

JL    2 


I  CONTENTS. 

Page 
celerated  Motion. — Velocity  of  Falling"  Bodies. — Momen- 
tum.— Action  and  Reaction  Equal. — Elasticity  of  Bodies. 
— Porosity  of  Bodies. — Reflected  Motion. — Angles  of  In- 
cidence and  Reflection. 


CONVERSATION  IV. 

ON    COMPOUND   MOTION.  69 

Compound  Motion,  the  result  of  two  opposite  forces. — Of 
Circular  Motion,  the  result  of  two  forces,  one  of  which 
confines  the  body  to  a  fixed  point. — Centre  of  motion, 
the  point  at  rest  while  the  other  parts  of  the  body  move 
round  it. — Centre  of  Magnitude,  the  middle  of  a  body. 
— Centripetal  force,  that  which  confines  a  body  to  a  fix- 
ed central  point. — Centrifugal  Force,  that  which  impels 
a  body  to  fly  from  the  centre. — Fall  of  Bodies  in  a  Pa- 
rabola.—Centre  of  Gravity,  the  Centre  of  Weight,  or 
point  about  which  the  parts  bs^ance  each  other. 


CONVERSATION  V. 

ON    THE   MECHANICAL   POWERS.  81 

Of  the  Power  of  Machines. — Of  the  Lever  in  general. — Of 
the  Lever  of  the  first  kind,  having  the  Fulcrum  between 
the  Power  and  the  weight.— Of  the  Lever  of  second  kind, 
having  the  Weight  between  the  power  and  the  Fulcrum. 
— Of  the  Lever  of  the  third  kind,  having  the  power  be- 
tween the  Fulcrum  and  the  Weight.  -^  Of  the  Pulley. 
— Of  the  Wheel  and  Axle.— Of  the  Inchned  Plane. — 
Of  the  Wedge.— Of  the  Screw. 


fiONTENTS.  Vli 

Page 
CONVERSATION  VI. 

ASTRUJi^OMY. 
CAUSES    OF    THE    EARTH's    AXI^UAL   MOTTOX.  107 

Of  the  Planets,  and  their  motion. —Of  the  Diurnal  motion 
of  the  Earth  and  Planets. 

CONVERSATION  VII. 

ON    THE    PLANETS.  121 

©f  the  Satellites  or  Moons. —  Gravity  diminishes  as  the 
Sqiiaie  of  the  Distance.— Of  the  Solar  System. — Of 
Comets. — Constellations,  sig"ns  of  the  Zodiac. — Of  Co- 
pernicus, Newton,  &c. 

CONVERSATION  VIII . 

ON    THE    EARTH.  lot 

Of  the  Terrestrial  Globe.— Of  the  Fig-ure  of  the  Earth.— Of 
the  Pendulum.— Of  the  Variation  of  the  Seasons,  and  of 
the  Length  'of  Days  and  Nights.  -  Of  the  Caiises  of  the 
Heat  of  Summer. — Of  Solar,  Siderial,  and  Equal  or 
Mean  Time. 

CONVERSATION  IX. 

ON    THE    MOOIN".  165 

Of  the  Moon^s  Motion. — Phases  of  the  Moon. — Eclip«;es 
of  the  Moon — Eclipses  of  Jupiter's  Moons. —Of  the  LAti- 


CONTENTS. 


Pag-e 
tilde  and  Longitude.— Of  the   Transits   of  the  Inferior 
Planets.— Of  the  Tides. 


CONVERSATION  X. 

HYDROSTATICS. 
ON    THE    MECHANICAL   PROPERTIES    OP   FLTJIDS.  177 

Definition  of  a  Fluid. — Distinction  between  Fkiids  and  Li- 
quids.— Of  Non-Elastic  Fluids,  scarcely  susceptible  of 
Compression. — Of  the  Cohesion  of  Fluids. — Of  their 
Gravitation.— Of  their  Eqiiihbrium.— Of  their  Pressure. 
—  Of  Specific  Gravity. — Of  the  Specific  Gravity  of  Bodies 
heavier  than  Water —  Of  those  of  the  same  weight  as 
Water. —Of  those  fighter  than  Water. — Of  the  Specific 
Gravity  of  Fluids. 

CONVERSATION  XL 

OP    SPRINGS,  FOUNTAINS,  &C.  193 

Of  the  Ascent  of  Vapour  and  the  Formation  of  Clouds. 
—Of  the  Formation  and  Fall  of  Rain,  &c. — Of  the  Forma- 
tion of  Springs. — Of  Rivers  and  Lakes. — Of  Fountains. 

CONVERSATION  XIL 

PNEUMATICS* 
ON    THE    MECHANICAL    PROPERTIES    OF    AIR.  205 

Of  the  Spring  or  Elasticity  of  the  Air.— Of  the  Weight 
of  the  Air. — Experiments  w'th  the  Air  Pum]). — Of  the 

Barometer. — Mode  of  Wejgliing  Air. Specific  Gravity 

of  Air.  — Of  Pumps. — Description  of  the  Sucking  Pump. 
— Description  of  the  Forcing  Pump. 


CONTENTS.  IX 

Page 
CONVERSATION  XIII. 

ON    WIND    AND    SOUND.  219 

Of  Wind  in  General.~Of  the  Trade  Wind.— Of  the  Pe- 
riodical Trade  Winds. — Of  the  Aerial  Tides.  -  Of  Sound 
in  General. — ^Of  Sonorous  Bodies. — Of  Musical  Sounds. 
Of  Concord  or  Harmony,  and  Melody. 

CON\^ERSATION  XIV. 

ON   OPTICS.        ^  237 

Of  Luminous,  Transparent,  and  Opaque  Bodies. — Of  the 
Radiation  of  Light.— Of  Shadows.— Of  the  Reflection  of 
Light. — Opaque  Bodies  seen  only  by  Reflected  Light. — 
Vision  Explitined. — Camera  Obscura. — ^^Image  of  Objects 
on  the  Retina. 

CONVERSATION  XV. 

ON  THE  ANGLE  OF  VISION,  AND  REFLECTION  OF  MIBROHS.     25S 

Angle  of  Vision. — Reflection  of  Plain  Mirrors. — Reflection 
of  Convex  Mirrors.— Reflection  of  Concave  Mirrors. 

CONVERSATION  XVI. 

ON  REFRACTION  AND  COLOURS.  271 

Transmission  of  Light  by  Transparent  Bodies.— Refrac- 
tion. -  Refraction  of  the  Atmosphere. — Refraction  of  a 
Lens. — Refraction  of  the  Prism. — Of  the  Colours  of  Rays 
©f  Light. — Of  the  Colours  of  Bodies. 


Page 
CONVERSATION  XVII. 

OPTICS.  292 

ON  THE  STRUCTUKE  OF  THE  EYE,  AND  OPTICAL  INSTRUMENT 

Description  of  the  Eye. — Of  the  Tmage  on  the  Retina. — 
Refraction  of  the  Humours  of  the  Eye. — Of  the  Use  of 
Spectacles. — Of  the  Single  Microscope. — Of  the  Double 
Microscope. —Of  the  Solar  Microscope. — Magic  Lan- 
tkorn. —Refracting  Telescope.— Reflecting  Telescope. 


DIRECTIONS 

FOR  FLdCIJVG  THE  EJ^GRAVIJSTGS, 

Plate      I.  to  face  page  39 

11.  -    -    -    -  66 

III.  -.  .  -   .  rs 

IV.  -   -   -   -  8a 

V.  -    -    -    -  93 

VI.  -    -    -    -  109 

VII.  ...     -  124 

Vlll.  .    -    -    -  129 

IX.  -    -    -    -  140 

X.  -    -     -     -  154 

XI.  -    -    -     -  159 

XII.  ....  166 

XIII.  .    .    -    -  180 

XIV.  ...    -  199 
XV.  .     -'   -    -  238 

XVI.  -     -    -     -  248 

XVM,  -    ...  261 

XVII  I.  ...    -  265 

XIX.  -     -     .    .  27-2 

XX.  ^    -    -     -  27-8 

XXI.  -     -     -     -  292 

XXH.  •    -     -    .  298 

XXUl.  .    .     -    -  302 


CONVERSATION  I 


UN  GENERAL  PROPERTIES  OF  BODIES. 

J  NTKODUCTION. GENETIAL    PROPERTIES    OF    BODIES. IMPENETHA- 

BILITY. EXTEJ«^SION. — FIGURE. DIVISIBILITY. INERTIA.-— 

ATTRACTION. ATTRACTION  OF  COHEStOX.— .DENSITY. RABITT. 

' — HEAT. ATTRACTION  OF  GRAVITATION. 


EMILY. 

I  MUST  request  your  assistance,  my  dear  Mrs.  B,, 
ill  a  charge  which  I  have  lately  undertaken  :  it  is  that 
of  instructing  my  youngest  sister,  a  task,  which  I  find 
proves  more  difficult  than  I  had  at  first  imagined.  I 
can  te^ch  her  the  common  routine  of  children's  lessons 
tolerably  well ;  but  she  is  such  an  inquisitive  little 
creature,  that  she  is  not  satisfied  without  an  explana- 
tion of  every  difficulty  that  occurs  to  her,  and  frequent- 
ly asks  me  questions  which  I  am  at  a  loss  to  answer. 
This  morning,  for  instance,  when  1  had  explained  to 
her  that  the  world  was  round  like  a  ball,  instead  of  be- 
ing flat  as  she  had  supposed,  and  that  it  was  surrounded 
by  the  air  she  asked  me  what  supported  it.  1  told  her 
that  it  required  no  support;  she  then  enquired  why  it 


14  GENERAL  PROPERTIES  OF  BODIES. 

did  not  fall  as  every  thing  else  did  ?  This  I  confess 
perplexed  me ;  for  1  had  myself  been  satisfied  with 
learning  that  the  world  floated  in  the  air,  without  con- 
sidering how  unnatural  it  was  that  so  heavy  a  body, 
bearing  the  weight  of  all  other  things,  should  be  able  to 
support  itself. 

Mrs»  B.  I  make  no  doubt,  my  dear,  but  that  I  shall 
be  able  to  explain  ihis  difficulty  to  you  ;  but  I  believe 
that  it  would  be  almost  impossible  to  render  it  intelligi- 
ble to  the  comprehension  of  so  young  a  child  as  your 
sister  Sophia.  You,  who  are  now  in  your  thirteenth 
year,  may,  I  think,  with  great  propriety,  learn  not  only 
the  cause  of  this  particular  fact,  but  acquire  a  general 
knowledge  of  the  laws  by  which  the  natural  world  is 
governed. 

Emily.  Of  all  things,  it  is  what  I  should  most  like 
to  learn;  but  I  was  afraid  it  was  too  difficult  a  study 
even  at  my  age. 

Mrs.  B.  Not  when  familiarly  explained :  if  you 
have  patience  to  attend,  I  will  most  willingly  give  you 
all  the  information  in  my  power.  You  may  perhaps, 
fmd  the  subject  rather  dry  at  first ;  but  if  I  succeed  in 
explaining  the  laws  of  nature,  so  as  to  make  you  under- 
stand them*,  I  am  sure  that  you  will  derive  not  only  in- 
struction, but  great  amusement  from  that  stud3^ 

Emily.  I  make  no  doubt  of  it,  Mrs.  B. ;  and  pray 
begin  by  explaining  why  the  earth  requires  no  support; 
for  that  is  the  point  which  just  now  most  strongly  ex- 
cites my  cu-riosity. 

Mrs.  B.  My  dear  Emily,  if  I  am  to  attempt  to  give 
you  a  general  idea  of  the  laws  of  nature,  which  is  no 
less  than  to  introduce  you  to  a  knowledge  of  the  science 


GENERAL  PROPEKTIES  OF  BODIES.  15 

i)f  natural  philosophy,  it  will  be  necessary  for  us  to  pro- 
ceed with  some  degree  of  regularity.  I  do  not  wish  to 
confine  you  to  the  systematic  order  of  a  scientific  trea- 
tise ;  but  if  we  were  merely  to  examine  every  vague 
question  that  may  chance  to  occur,  our  progress  would 
.  be  but  very  slow.  Let  us,  therefore,  begin  by  taking  a 
short  survey  of  the  general  properties  of  bodies,  some  of 
which  must  necessarily  be  explained  before  I  can  at- 
tempt to  make  you  understand  why  the  earth  requires 
no  support. 

When  I  speak  of  bodies,  I  mean  substances,  of  what- 
ever nature,  whether  solid  or  fluid  ;  and  matter  is  the 
general  term  used  to  denote  the  substance,  whatever 
its  nature  be,  of  which  the  different  bodies  are  composed. 
Thus,  wood  is  the  matter  of  which  this  table  is  made ; 
water  is  the  matter  with  which  this  glass  is  filled,  &c. 

Emily,  I  am  very  glad  .you  have  explained  the 
I  meaning  of  the  word  matter,  as  it  has  corrected  an  er- 
roneous conception  I  had  formed  of  it :  I  thought  that 
it  was  applicable  to  solid  bodies  only. 

Mrs.  B,  There  are  certain  properties  which  appear 
to  be  common  to  all  bodies,  and  are  hence  called  the 
essential  properties  of  bodies;  these  are,  hnpenetra- 
hility.  Extension,  Figure,  Divisibility,  Inertia,  and 
dttraction.  These  are  called  the  general  properties  of 
bodies,  as  we  do  not  suppose  any  body  to  exist  without 
them. 

By  impenetrability,  is  meant  the  property  which  bo- 
dies have  of  occupying  a  certain  space,  so  that,  where 
one  body  is,  another  cannot  be,  without  displacing  the 
former ;  for  two  bodies  cannot  exist  in  the  same  place 


16  GENERAL  PROPERTIES  OF  BODIES. 

at  the  same  time.  A  liquid  may  be  more  easily  remo- 
ved than  a  solid  body ;  jet  it  is  not  the  less  substantial 
since  it  is  as  impossible  for  a  liquid  and  a  solid  to  occu- 
py the  same  space  at  the  same  time,  as  for  two  solid  bo- 
dies to  do  so.  For  instance,  if  you  put  a  spoon  into  a 
glass  full  of  water,  the  water  will  flow  over  to  make 
room  for  the  spoon. 

Emily,  I  understand  this  perfectly.  Liquids  are  in 
reality  as  substantial  or  as  impenetrable  as  solid  bodies, 
and  they  appear  less  so,  only  because  they  are  more 
easily  displaced. 

Mrs.  B.  The  air  is  a  fluid  differing  in  its  nature 
from  liquids,  but  no  less  impenetrable.  If  I  endeavour 
to  fill  this  phial  by  plunging  it  into  this  bason  of  water, 
the  air,  you  see,  rushes  out  of  the  phial  in  bubbles,  in 
order  to  make  way  for  the  water,  for  the  air  and  the 
water  cannot  exist  together  in  the  ^me  space,  any 
more  than  two  hard  bodies  ;  and  if  1  reverse  this  goblet, 
and  plunge  it  perpendicularly  into  the  water,  so  that 
the  air  will  not  be  able  to  escape,  the  water  will  no  lon- 
ger be  able  to  fdl  the  goblet. 

Emily,  But  it  rises  a  considerable  way  into  the 
glass. 

Mrs,  B,  Because  the  water  compresses  or  squeezes 
the  air  into  a  small  space  in  the  upper  part  of  the  glass  ; 
but,  as  long  as  it  remains  there,  no  other  body  can  oc- 
cupy the  same  place. 

Emily,  A  difficulty  has  just  occurred  to  me,  with 
regard  to  the  impenetrability  of  solid  bodies;  if  a  nail 
is  driven  into  a  piece  of  wood,  it  penetrates  it,  and  both 
the  wood  and  the  nail  occupy  the  same  &pace  that  the 
wood  alone  did  before  ? 


iiENERAL  PROPERTIES  OF  BODIESv  17 

Mrs,  B,  The  nail  penetrates  between  the  particles 
of  the  wood,  by  forcing  them  to  make  waj  for  it ;  for 
you  know  that  not  a  single  atom  of  wood  can  remain  in 
the  space  which  the  nail  occupies ;  and  if  the  wood  is 
not  increased  in  size  by  the  addition  of  the  nail,  it  is 
because  wood  is  a  porous  substance,  like  sponge,  the 
particles  of  which  may  be  compressed  or  squeezed  clo- 
ser together ;  and  it  is  thus  that  they  make  way  for  the 
nail. 

We  may  now  proceed  to  the  next  general  property 
of  bodies,  ecctension,  A  body  which  occupies  a  certain 
space  must  necessarily  have  extension  ',  that  is  to  say, 
lengthy  breadth,  and  depths  these  are  called  the  di- 
mensions of  extension :  can  you  form  an  idea  of  any 
body  without  them  ? 

Emily,  No ;  certainly  I  cannot ;  though  these  di- 
mensions must,  of  course,  vary  extremely  in  different 
bodies.  The  length,  breadth,  and  depth  of  a  box,  or  of 
a  thimble,  are  very  different  from  those  of  a  walking- 
stick,  or  of  a  hair. 

But  is  not  height  also  a  dimension  of  extension  ? 

Mrs,  B,  Height  and  depth  are  the  same  dimension, 
considered  in  different  points  of  view  ;  if  you  measure 
a  body,  or  a  space,  from  the  top  to  the  bottom,  you  call 
it  depth ;  if  from  the  bottom  upwards, you  call  it  height ; 
thus  the  depth  and  height  of  a  box  are,  in  fact,  the  same 
thing. 

Emily,  Very  true ;  a  moment's  consideration  would 
have  enabled  me  to  discover  that ;  and  breadth  and 
width  are  also  the  same  dimension. 

Mrs,  B.  Yes ;  the  limits  of  extension  constitute 
jigiire  or  shape.    You  conceive  that  a  body  having 


18  GENERAL  PROPERTIES  OF  BODIES. 

length,  breadth,  and  depth,  cannot  be  without  form,  ti^ 
ther  symmetrical  or  irregular  ? 

Emily,  Undoubtedly ;  and  this  ppoperty  admits  of 
almost  an  infinite  variety.. 

Mrs,  B,  Nature  has  assigned  regular  forms  to  her 
productions  in  general.  The  natural  form  of  mineral 
substances  is  that  of  crystals,  of  which  there  is  a  great 
variety.  Many  of  them  are  very  beautiful,  and  no  less 
remarkable  by  their  transparency,  or  colour,  than  by 
the  perfect  regularity  of  their  forms,  as  may  be  seen  in 
the  various  museums  and  collections  of  natural  history. 
The  vegetable  and  animal  creation  appears  less  symme- 
trical, but  is  still  more  diversified  in  figure  than  the  mi- 
neral kingdom.  .  Manufactured  substances  assume  the 
various  arbitrary  forms  which  the  art  of  man  designs 
for  them ;  and  an  infinite  number  of  irregular  forms 
are  produced  by  fractures,  and  by  the  dismemberment 
of  the  parts  of  bodies. 

Emily.     Such  as  a  piece  of  broken  china,  or  glass? 

Mrs.  B.  Or  the  fragments  of  mineral  bodies  which 
are  broken  in  being  dug  out  of  the  earth,  or  decayed  by 
the  effect  of  torrents  and  other  causes.  The  picturesque 
effect  of  rock-scenery  is  in  a  great  measure  owing  to 
accidental  irregularities  of  this  kind. 

We  may  now  proceed  to  divisibility  ;  that  is  to  saj', 
a  susceptibility  of  being  divided  into  an  indefinite  num- 
ber of  parts.  Take  any  small  quantity  of  matter,  a  grain 
of  sand  for  instance,  and  cut  it  into  two  parts  ;  these 
two  parts  might  be  again  divided,  had  we  instruments 
sufficiently  fine  for  the  purpose ;  and  if,  by  means  of 
pounding,  grinding,  and  other  similar  methods,  we  car- 
ry this  division  to  the  greatest  possible  extent,  and  re- 


GENERAL- PROPERTIES  OF  BODIES.  19 

duc5  the  body  to  its  finest  imaginable  particles,  yet  not 
one  of  the  particles  v/ill  be  destroyed,  and  the  body  will 
continue  to  exist,  though  in  this  altered  state. 

The  melting  of  a  solid  body  in  a  liquid  affords  a  very 
striking  example  of  the  extreme  divisibility  of  matter  ; 
when  you  sweeten  a  cup  of  tea,  for  instance,  with  what 
minuteness  the  sugar  must  be  divided  to  be  diffused 
throughout  the  whole  of  the  liquid. 

Emily.  And  if  you  pour  a  few  drops.of  red  wine  in- 
to a  glass  of  water,  they  immediately  tinge  the  whole  of 
the  water,  and  must  therefore  be  diffused  throughout  it. 

Mrs,  B,  Exactly  so ;  and  the  perfume  of  this  la- 
vender water  will  be  almost  as  instantaneously  diffused 
throughout  the  room,  if  I  take  out  the  stopper. 

Emily,  But  in  this  case  it  is  only  the  perfume  of  the 
lavender,  and  not  the  water  itself,  that  is  diffused  in  the 
room  ? 

Mrs,  B,  The  odour  or  smell  of  a  body  is  part  of  the 
body  itself,  and  is  produced  by  very  minute  particles  or 
exhalations  which  escape  from  odoriferous  bodies.  It 
would  be  impossible  that  you  should  smell  the  lavender- 
water,  if  particles  of  it  did  not  come  in  actual  contact 
with  your  nose. 

Emily,  But  when  I  smell  a  flower,  I  see  no  vapour 
rise  from  it ;  and  yet  I  can  perceive  the  smell  at  a  con- 
siderable distance. 

Mrs,  B,  You  could,  I  assure  you,  no  more  smell  a 
flower,  the  odoriferous  particles  of  which  did  not  touch 
your  nose,  than  you  could  taste  a  fruit,  the  flavoured 
particles  of  which  did  not  come  in  contact  with  your 
tongue. 

Emily,    That  is  w^ouderful  indeed;   the  particles 


2»  GENERAL  PROPERTIES  OV  BODIES* 

then,  which  exhale  from  the  flower  and  from  the  la^ea-. 
der-water,  are,  I  suppose,  too  small  to  be  visible  ? 

Mrs,  B,  Certainly:  you  may  form  some  idea  of 
their  extreme  minuteness,  from  the  immense  number 
which  must  have  escaped  in  order  to  perfume  the  whole 
room ;  and  yet  there  is  no  sensible  diminution  of  the 
liquid  in  the  phial. 

Emily,  But  the  quantity  must  really  be  diminish- 
ed ?  • 

Mrs,  B,  Undoubtedly  ;  and  were  you  to  leave  the 
bottle  open  a  sufficient  length  of  time,  the  whole  of  the 
water  would  evaporate  and  disappear.  But  though  so 
minutely  subdivided  as  to  be  imperceptible  to  any  of  our 
senses,  each  particle  would  continue  to  exist ;  for  it  is 
not  within  the  power  of  man  to  destroy  a  single  particle 
of  matter:  nor  is  there  any  reason  to  suppose  that  in 
nature  an  atom  is  ever  annihilated. 

Emily,  Yet,  when  a  body  is  burnt  to  ashes,  part  of 
it,  at  least,  appears  to  be  effectually  destroyed  ^  Look 
how  small  is  the  residue  of  ashes  beneath  the  grate, 
from  all  the  coal's  which  have  been  consumed  within  it. 

Mrs,  B,  That  part  of  the  coals,  which  you  suppose 
to  be  destroyed,  evaporates  in  the  form  of  smoke  and 
vapour,  whilst  the  remainder  is  reduced  to  ashes.  A 
body,  in  burning,  undergoes  no  doubt  very  remarkable 
changes ;  it  is  generally  subdivided  ;  its  form  and  co- 
jour  altered  ;  its  extension  increased  :  but  the  various 
parts,  into  which  it  has  been  separated  by  combustion, 
continue  in  existence,  and  retain  all  the  essential  pro- 
perties of  bodies. 

Emily,  But  that  part  of  a  burnt  body  which  evapo- 
rates in  smoke  has  no  figure  ;  smoke,  it  is  true,  ascends 


GENERAL  PROPERTIES  OF  BODIES,  21 

ill  columns  into  the  air,  but  it  is  soon  so  much  diflfused 
as  to  lose  all  form  ;  it  becomes  indeed  invisible. 

Mvi.  B,  Invisible,  I  allow  ;  but  we  must  not  ima- 
gine that  what  we  no  longer  see  no  longer  exists.  \\  ere 
every  particle  of  matter  that  becomes  invisible  annihi- 
lated, the  world  itself  would  in  the  course  of  time  be 
destroyed.  The  particles  of  smoke,  when  diflfused  in 
the  air,  continue  still  to  be  particles  of  matter,  as  well 
as  when  more  closely  united  in  the  form  of  coals :  they 
are  really  as  substantial  in  the  one  state  as  in  the  other, 
and  equally  so  when  by  their  extreme  subdivision  they 
become  invisible.  No  particle  of  matter  is  ever  destroy- 
ed :  this  is  a  principle  you  must  constantly  remember. 
Every  thing  in  nature  decays  and  corrupts  in  the  lapse 
of  time.  We  die,  and  our  bodies  moulder  to  dust;  but 
not  a  single  atom  of  them  is  lost;  they  serve  to  nourish 
the  earth,  whence,  while  living,  they  drew  their  sup- 
port. 

The  next  essential  property  of  matter  is  called  iner- 
tia ;  this  word  expresses  the  resistance  which  inactive 
matter  makes  to  a  change  of  state.  Bodies  appear  to 
be  equally  incapable  of  changing  their  actual  state, 
whether  it  be  of  motion  or  of  rest.  You  know  that  it 
requires  force  to  put  a  body  which  is  at  rest  in  motion  ; 
an  exertion  of  strength  is  also  requisite  to  stop  a  body 
which  is  already  in  motion.  The  resistance  of  the  bo- 
dy to  a  change  of  state,  in  either  case,  is  called  its 
inertia. 

Emily.  In  playing  at  base-ball  I  am  obliged  to  use 
all  my  strength  to  give  a  rapid  motion  to  the  ball ;  and 
when  I  have  to  catch  it,  I  am  sure  1  feel  the  resistance 
it  makes  to  being  stopped.  But  if  I  did  not  catch  it,  it 
would  soon  fall  to  the  ground  and  stop  of  itself. 


^2  GENERAL  PROPERTIES  OF  BODIES. 

'  Mrs,  B.  Inert  matter  is  as  incapable  of  stopping  of 
itself,  as  it  is  of  putting  itself  into  motion  :  when  the 
ball  ceases  to  move,  therefore,  it  must  be  stopped  by 
some  other  cause  or  power ;  but  as  it  is  one  with  which 
you  are  yet  unacquainted,  we  cannot  at  present  invest 
tigate  its  effects. 

The  last  property  which  appears  to  be  common  to  all 
bodies  is  attraction.  All  bodies  consist  of  infinitely  small 
particles  of  matter,  each  of  which  possesses  the  power 
of  attracting  or  drawing  towards  it,  and  uniting  with 
any  other  particle  sufficiently  near  to  be  within  the  in- 
fluence of  its  attraction ;  but  in  minute  particles  this 
power  extends  to  so  very  small  a  distance  around  them 
that  its  effect  is  not  sensible,  unless  they  are  (or  at  least 
appear  to  be)  in  contact ;  it  then  makes  them  stick  or 
adhere  together,  and  is  hence  called  the  attraction  of 
cohesion.  Without  this  power,  solid  bodies  would  fall 
in  pieces,  or  rather  crumble  to  atoms. 

Emily,  T  am  so  much  accustomed  to  see  bodies  firm 
and  solid,  that  it  never  occurred  to  me  that  any  power 
was  requisite  to  unite  the  particles  of  which  they  are 
composed.  But  the  attraction  of  cohesion  does  not,  I 
suppose,  exist  in  liquids ;  for  the  particles  of  liquids  do 
not  remain  together  so  as  to  form  a  body,  unless  con- 
fined in  a  vessel  ? 

Mr 9,  B,  I  beg  your  pardon  ;  it  is  the  attraction  of 
cohesion  which  holds  this  drop  of  water  suspended  at 
the  end  of  my  finger,  and  keeps  the  minute  watery  par- 
ticles of  which  it  is  composed  united.  But  as  this 
power  is  stronger  in  proportion  as  the  particles  of  bo- 
dies are  more  closely  united,  the  cohesive  attraction  of 
solid  bodies  is  much  greater  thaa  that  of  fluids.. 


GENERAL  PROPERTIES  OF  BODIES.  23 

The  thianer  and  lighter  a  fluid  is,  the  less  is  the  co- 
hesive attraction  of  its  particles,  because  thej  are  fur- 
ther apart;  and  in  elastic  fluids,  such  as  air,  there  is  na 
cohesive  attraction  among  the  particles. 

Emily,  That  is  very  fortunate  ;  for  it  would  be  im- 
possible to  breathe  the  air  in  a  solid  mass ;  or  even  in  a 
liquid  state. 

But  is  the  air  a  body  of  the  same  nature  as  other  bo- 
dies ? 

Mrs.  B,    Undoubtedly,  in  all  essential  properties. 

Emily,  Yet  you  say  that  it  does  not  possess  one  of 
the  general  properties  of  bodies — cohesive  attraction  ? 

Mrs,  B,  The  particles  of  air  are  not  destitute  of  the 
power.of  attraction,  but  they  are  too  far  distant  from 
each  other  to  be  influenced  by  it ;  and  the  utmost  efforts 
of  human  art  have  proved  ineffectual  in  tlie  attempt  to 
compress  them,  so  as  to  bring  them  vy^ithin  the  sphere  of 
each  other's  attraction,  and  make  them  cohere. 

Emily,  If  so,  how  is  it  possible  to  prove  tliat  they  are 
endowed  with  this  power? 

Mrs,  B,  The  air  is  formed  of  particles  precisely  of 
the  same  nature  as  those  which  enter  into  the  compo- 
sition of  liquid  and  solid  bodies,  in  which  state  we  have 
a  proof  of  their  attraction.  ^ 

Emily.  It  is  then,  I  suppose,  owing  to  the  different 
degrees  of  attraction  of  different  substances,  that  they 
are  hard  or  soft ;  and  that  liquids  are  thick  or  thin  ? 

Mrs,  B,  Yes;  but  you  would  express  your  meaning 
better  by  the  term  density,  which  denotes  the  degree  of 
closeness  and  compactness  of  the  particles  of  a  body : 
thus  you  may  say,  both  of  solids,  and  of  liquids,  that  the 
stronger  the  cohesive  attraction,  the  greater  is  the  deu- 


24  GENERAL  PROPERTIES  OF  BOBIES. 

sity  of  the  body.  In  philosophical  language,  density  is 
said  to  be  that  property  of  bodies  by  which  they  contain 
a  certain  quantity  of  matter,  under  a  certain  bulk  or 
magnitude.  Rarity  is  the  contrary  of  density ;  it  de- 
notes the  thinness  and  subtlety  of  bodies :  thus  you 
Arould  say  that  mercury  or  quicksilver  was  a  very  dense 
fluid ;  ether,  a  very  rare  one,  &c. 

Caroline*  But  how  are  we  to  judge  of  the  quantity 
of  matter  contained  in  a  certain  bulk  ? 

Mrs,  B.  By  the  weight :  under  the  same  bulk  bo- 
tlies  are  said  to  be  dense  in  proportion  as  they  are 
heavy. 

Emily.  Then  we  may  say  that  metals  are  dense 
bodies,  wood  comparatively  a  rare  one,  &c.  But,  Mrs. 
B.,  when  the  particles  of  a  body  are  so  near  as  to  at- 
tract each  other,  the  effect  of  this  power  must  increase 
as  they  are  brought  hy  it  closer  together ;  so  that  one 
would  suppose  that  the  body  would  gradually  augment 
in  density,  till  it  was  impossible  for  its  particles  to  be 
more  closely  united.  Now,  we  know  that  this  is  not 
the  case  ;  for  soft  bodies,  such  as  cork,  sponge,  or  but- 
ter, never  become,  in  consequence  of  the  increasing  at- 
traction of  their  particles,  as  hard  as  iron? 

Mrs.  B.  In  such  bodies  as  cork  and  sponge,  the 
particles  which  come  in  contact  are  so  few  as  to  pro- 
duce but  a  slight  degree  of  cohesion  :  they  are  porous 
bodies,  which,  owing  to  the  peculiar  arrangement  of 
their  particles,  abound  with  interstices  which  separate 
the  particles ;  and  these  vacancies  are  filled  with  air, 
the  spring  or  el??sticity  of  which  prevents  the  closer 
union  of  the  parts.  But  there  is  another  fluid  much 
more  subtle  than  air,  which  pervades  all  bodies,  this  is 


GENERAL  PROPERTIES  OF  BODIES.  25 

heat  Heat  insinuates  itself  more  or  less  between  the 
particles  of  all  bodies,  and  forces  them  asunder;  you 
may  therefore  consider  heat,  and  the  attraction  of  cohe- 
sion, as  constantly  acting  in  opposition  to  each  other. 

Emily.      The  one  endeavouring  to  rend  a  body  to 
pieces,  the  other  to  keep  its  parts  firmly  united. 

Mrs,  B,  And  it  is  this  struggle  between  the  conten- 
ding forces  of  heaf  and  attraction,  which  prevents  the 
extreme  degree  of  density  which  would  result  from  the 
sole  influence  of  the  attraction  of  cohesion. 

Emily.  The  more  a  body  is  heated  then,  the  more 
its  particles  will  be  separated. 

Mrs.  B,  Certainly ;  we  find  that  bodies  swell  or 
dilate  by  heat:  this  effect  is  very  sensible  in  butter,  for 
instance,  which  expands  by  the  application  of  heat,  till 
at  length  the  attraction  of  cohesion  is  so  far  diminished 
that  the  particles  separate,  and  the  butter  becomes  li- 
quid. A  similar  effect  is  produced  by  heat  on  metals, 
and  all  bodies  susceptible  of  being  melted.  Liquids, 
yon  know,  are  made  to  boil  by  the  application  of  heat; 
I  the  attraction  of  cohesion  then  yields  entirely  to  the  ex- 
pansive power;  the  particles  are  totally  separated  and 
converted  into  steam  or  vapour.  But  the  agency  of  heat 
is  in  no  body  more  sensible  than  in  air,  which  dilates 
and  contracts  by  its  increase  or  diminution  in  a  very 
remarkable  degree. 

Emily.  The  effects  of  beat  appear  to  be  one  of  the 
most  interesting  parts  of  natural  philosophy. 

Mrs.  B.  That  is  true;  but  heat  is  so  intimately 
connected  with  chemistry,  that  you  must  allow  me  to 
defe  the  investigation  of  its  properties  till  you  become 
a;cquainted  with  that  science. 


26  GENERAL  PROPERTIES  OF  BODIES. 

To  return  to  its  antagonist,  the  attraction  of  cohe- 
sion ;  it  is  this  power  which  restores  to  vapour  its  liquid 
form,  which  unites  it  into  drops  when  it  fails  to  the 
earth  in  a  shower  of  rain,  which  gathers  the  dew  into 
brilliant  gems  on  the  blades  of  grass. 

Emily.  And  I  have  often  observed  that  after  a  show- 
er, the  water  collects  into  large  drops  on  the  leaves  of 
plants;  but  I  cannot  saj  that  I  perfectly  understand 
how  the  attraction  of  cohesion  produces  this  effect. 

Mrs.  B.  Rain  does  not  fall  from  the  clouds  in  the 
form  of  drops,  but  in  that  of  mist  or  vapour,  which  is 
composed  of  very  small  watery  particles  ;  these  in  their 
descent,  mutually  attract  each  other,  and  those  that  are 
sufficiently  near  in  consequence  unite  and  form  a  drop, 
and  thus  the  mist  is  transformed  into  a  shower.  The 
dew  also  was  originally  in  a  state  of  vapour,  but  is,  by 
the  mutual  attraction  of  the  particles,  formed  into  small 
globules  on  the  blades  of  grass:  in  a  similar  manner  the 
rain  upon  the  leaf  collects  into  large  drops,  which  when 
they  become  too  heavy  for  the  leaf  to  support  fall  to  the 
ground. 

Emily.  All  this  is  wonderfully  curious !  I  am  al- 
;Qiost  bewildered  with  surprise  and  admiration  at  the 
number  of  new  ideas  I  have  already  acquired. 

Mrs.  B.  Every  step  that  you  advance  in  the  pursuit  of 
natural  science,  will  fill  your  mind  with  admiration  and 
gratitude  towards  its  Divine  Author.  In  the  study  of 
nafiiral  philosophy,  we  must  consider  ourselves  as  read- 
ing the  book  of  nature,  in  which  the  bouatiful  goodness 
anl  wisdom  of  Godis  revealed  to  all  mankind  ;  no  stu- 
dy can  then  i^.m\  more  to  \f\Y\{y  the  heart,  and  raise  it 
to  a  religious  contomplatiun  of  the  Divine  perfections. 


GENERAL  PROPERTIES  OF  BODIES.  27. 

There  is  another  curious  effect  of  the  attraction  of 
cohesion  which  I  must  point  out  to  you.  It  enables  li- 
quids to  rise  above  their  level  in  capillary  tubes: 
these  are  tubes,  the  bores  of  which  are  so  extremely 
small  that  liquids  ascend  within  them,  from  the  cohe- 
sive attraction  between  the  particles  of  the  liquid  and 
the  interior  surface  jof  the  tube.  Do  you  perceive  the 
water  rising  above  its  level  in  this  small  glass  tube> 
which  I  have  immersed  in  a  goblet  full  of  water? 

Emily.  Oh  yes  ;  I  see  it  slowly  creeping  up  the  tube, 
but  now  it  is  stationary  :  will  it  rise  no  higher  ? 

Mrs.  B.  No ;  because  the  cohesive  attraction  be- 
tween the  water  and  the  internal  surface  of  the  tube  is 
now  balanced  by  the  weight  of  the  water  within  it:  if 
the  bore  of  the  tube  were  narrower  the  water  would 
rise  higher;  and  if  you  imivierse  several  tubes  of  bores 
of  different  sizes,  you  will  see  it  rise  to  different  heights 
in  each  of  them.  In  making  this  experiment,  you  should 
colour  the  water  with  a  little  red  wine,  in  order  to  ren- 
der the  effect  more  obvious. 

All  porous  substances,  such  as  sponge,  bread,  linen, 
&c.  may  be  considered  as  collections  of  capillary  tubes : 
if  you  dip  one  end  of  a  lump  of  sugar  into  water  the 
water  will  rise  in  it,  and  wet  it  considerably  above  the 
surface  of  that  into  which  you  dip  it. 

Emily.  In  making  tea  I  have  often  observed  that 
effect,  without  being  able  to  account  for  it. 

Mrs.  B.  Now  that  you  are  acquainted  with  the  at- 
traction of  cohesion,  I  must  endeavour  to  explain  to 
you  that  of  Gravitation^  which  is  a  modification  of  the 
same  power  ;  the  first  is  perceptible  only  in  very  minute 


28  GENERAL  PROPERTIES  OP  BODIES. 

particles,  and  at  very  small  distances  ;  the  other  acts  on 
the  largest  bodies,  and  extends  to  immense  distances. 

Emily,  You  astonish  me :  surely  jou  do  not  mean  to 
say  that  large  bodies  attract  each  other. 

Mrs,  B.  Indeed  I  do:  let  us  take,  for  example, 
one  of  the  largest  bodies  in  nature,  and  observe  whe- 
ther it  does  not.  attract  other  bodies.  What  is  it  that 
occasions  the  fall  of  this  book,  when  I  no  longer  sup- 
port it  ? 

Emihj,  Can  it  be  the  attraction  of  the  earth  ?  I  thought 
that  all  bodies  had  a  natural  tendency  to  fall. 

.Mrs.  B,  They  have  a  natural  tendency  to  fall,  it  is 
true  ;  but  that  tendency  is  produced  entirely  by  the  at- 
traction of  the  earth :  the  earth  being  so  much  larger 
than  any  My  on  its  surface,  forces  every  body,  which 
is  not  supported,  to  fall  upon  it. 

Endli;.  If  the  ttVxdtncy  which  bodies  Imve  to  fall  re- 
sults from  the  earth's  attractive  power,  the  earth  itself 
can  have  no  such  tendency,  since  it  cannot  attract  it 
self,  and  therefore  it  requires  no  support  to  prevent  it 
from  falling.  Yet  the  idea  that  bodies  do  not  fall  of  their 
own  accord,  but  that  they  are  drawn  towards  the  earth 
by  its  attraction,  is  so  new  and  strange  to  me,  that  I 
know  not  how  to  reconcile  myself  to  it. 

Mrs,  B,  When  you  are  accustomed  to  consider  the 
fall  of  bodies  as  depending  on  this  cause,  it  will  appear 
to  you  as  natural,  and  surely  much  more  satisfactory, 
than  if  the  cause  of  their  tendency  to  fall  were  totally 
unknown.  Thus  you  understand,  that  all  matter  is 
attractive,  from  the  smallest  particle  to  the  largest 
mass ;  and  that  bodies  attract  each  other  with  a  force 
proportional  to  tlie  quantity  of  matter  they  contain. 


liEMERAL  PKOPEIHIES  OF  BODIES.  29 

Emily,  I  do  not  perceive  any  urfference  between 
the  attraction  of  cohesion  and  that  of  gravitation  :  is  it 
not  because  every  particle  of  matter  is  endowed  with 
an  attractive  power,  that  large  bodies,  consisting  of  a 
great  number  of  particles,  are  so  strongly  attractive? 

Mrs,  B,  True.  There  is,  however,  this  difference 
between  the  attraction  of  particles  and  that  of  masses, 
that  the  former  is  stronger  than  the  latter,  in  proportion 
to  the  quantity  of  matter.  Of  this  you  have  an  instance 
in  the  attraction  of  capillary  tubes,  in  wliich  liquids 
ascend  by  the  attraction  of  cohesion,  in  opposition  to 
that  of  gravity.  It  is  on  this  account  that  it  is  necessary 
that  the  bore  of  the  tube  should  be  extremely  small;  for 
if  tlie  column  of  water  within  the  tube  is  not  very  mi- 
nute, the  attraction  would  not  be  able  either  to  raise  or 
support  its  weight,  in  opposition  to  that  of  gravity. 

You  may  observe,  also,  that  all  solid  bodies  are  en- 
abled by  the  force  of  the  cohesive  attraction  of  their  par- 
ticles to  resist  that  of  gravity,  which  would  otherwise 
disunite  them,  and  bring  them  to  a  level  with  the  ground, 
as  it  does  in  the  case  of  liquids,  the  cohesive  attraction 
of  which  is  not  sufficient  to  enable  it  to  resist  the  power 
of  gravity. 

Emily.  And  some  solid  bodies  appear  to  be  of  this 
nature,  as  sand  and  powder  for  instance  :  there  is  no  at- 
4;raction  of  cohesion  between  their  particles  ? 

■Mrs,  B,  Every  grain  of  powder  or  sand  is  composed 
of  a  great  number  of  other  more  minute  particles,  firmly 
united  by  the  attraction  of  cohesion  ;  but  amongst  the 
separate  grains  there  is  no  sensible  attraction,  because 
they  are  not  in  sufficiently  close  contact. 

Emily,    Yet  they  actually  touch  each  other  P 
c2 


30  GENERAL  PROPERTIES  OF  BOi)IES. 

Mrs,  B,  The  surface  of  bodies  is  in  general  so  rough 
and  uneven,  that  when  in  actual  contact,  they  touck 
each  other  only  by  a  few  points.  Thus,  if  I  lay  upon  the 
table  this  book,  the  binding  of  which  appears  perfectly 
smooth,  yet  so  few  of  the  particles  of  its  under  surface 
come  in  contact  with  the  table,  that  no  sensible  degree 
of  cohesive  attraction  takes  place  ;  for  you  see,  that  it 
does  not  stick,  or  cohere  to  the  table,  and  I  find  no  diffi- 
culty in  lifting  it  off. 

It  is  only  when  surfaces  perfectly  flat  and  well  po- 
lished are  placed  in  contact,  that  the  particles  approach 
in  sufficient  number,  and  closely  enough,  to  produce  a 
sensible  degree  of  cohesive  attraction.  Here  are  two 
hemispheres  of  polished  metal,  I  press  their  flat  surfa- 
ces together,  having  previously  interposed  a  few  drops 
of  oil,  to  fill  up  every  little  porous  vacancy.  Now  try 
to  separate  them. 

Emily,  It  requires  an  effort  beyond  my  strength, 
though  there  are  handles  for  the  purpose  of  pulling  them 
asunder.  Is  the  firm  adhesion  of  the  two  hemispheres, 
merely  owing  to  the  attraction  of  cohesion  ? 

Mrs,  B,  There  is  no  force  more  powerful,  since  it  is 
by  this  that  the  particles  of  the  hardest  bodies  are  held 
together.  It  would  require  a  weight  of  several  pounds, 
to  separate  these  hemispheres. 

Emily,  In  making  a  kaleidoscope,  I  recollect  that 
the  two  plates  of  glass,  which  were  to  serve  as  mirrors, 
stuck  so  fast  together,  that  I  imagined  some  of  the  gum 
I  had  been  using  had  by  chance  been  interposed  be- 
tween them ;  but  now  I  make  no  doubt  but  that  it  was 
their  own  natural  cohesive  attraction  which  produced 
this  effect. 


GENERAL  PROPERTIES  OF  BODIES.  3^ 

Mrs.  B.  Very  probably  it  was  so ;  for  plate-glass 
has  an  extremely  smooth,  flat  surface,  admitting  of  the 
contact  of  a  great  number  of  particles,  between  two 
plates,  laid  one  over  the  other. 

Emily,  But  Mrs.  B.,  the  cohesive  attraction  of  some 
bodies  is  much  greater  than  that  of  others  ;  thus  glue, 
gum,  and  paste,  cohere  with  singular  tenacity. 

Mrs,  B,  That  is  owing  to  the  peculiar  chemical  pro- 
perties of  those  bodies,  independently  of  their  cohesive 
attraction. 

There  are  some  other  kinds  of  modifications  of  at- 
traction peculiar  to  certain  bodies ;  namely,  that  of 
magnetism,  and  of  electricity ;  but  we  shall  confine 
our  attention  merely  to  the  attraction  of  cohesion  and 
of  gravity;  the  examination  of  the  latter  we  shall 
resume  at  our  next  meeting. 


CONVERSATION  11. 


ON  THE  ATTRACTION  OF  GRAVITY. 

ATTBACTION    OF    GRAVITATION,     CONTINUED OF    WEIGHT. — —OF 

TBE   FALL    OF    BODIES. OF    TH£    RUSISTANCE    OF    THE     AIR. OF 

THE   ASCENT   OF   LIGHT    flODlES. 


I  HAVE  related  to  nay  sister  Caroline  all  that  jou 
have  taught  me  of  natural  philosophy,  and  she  has  been 
so  much  delighted  by  it,  that  she  hopes  you  will  have 
the  goodness  to  admit  her  to  your  lessons. 

Mrs,  B,  Very  willinglj'- ;  but  I  did  not  think  you 
had  any  taste  for  studies  of  this  nature,  Caroline  ? 

Caroline.  I  confess,  Mrs.  B.,  that  hitherto  I  had 
formed  no  very  agreeable  idea,  either  of  philosophy,  or 
philosophers ;  but  what  Emily  has  told  me,  has  excited 
my  curiosity  so  much,  that  1  shall  be  highly  pleased  if 
you  will  allow  me  to  become  one  of  your  pupils.  < 

Mrs,  B,  I  fear  that  I  shall  not  find  you  so  tractable  a 
scholar  as  Emily  ;  I  know  that  you  are  much  biassed  in 
favour  of  your  own  opinions. 


o4  O^  THE  ATTRACTION  OF  GUAVITV. 

Caroline,  Then  you  will  have  the  greater  merit  in 
reforming  them,  Mrs.  B. ;  and  after  all  the  wonders 
that  Emily  has  related  to  me,  I  think  1  stand  but  little 
chance  against  you  and  your  attractions. 

Mrs,  B,  You  will,  I  doubt  not,  advance  a  number  of 
objections;  but  these  I  shall  willingly  admit,  as  ihey 
will  be  a  means  of  elucidating  the  subject.  Emily,  do 
you  recollect  the  names  of  the  general  properties  of 
bodies? 

Eml^y,  Impenetrability,  extension,  figure,  divisibi- 
lity, inertia,  and  attraction. 

Mrs,  B,  Very  well.  You  must  remember  that  these 
are  properties  common  to  all  bodies,  and  of  which  they 
cannot  be  deprived ;  all  other  properties  of  bodies  are 
called  accidental,  because  they  depend  on  the  relation 
on  connection  of  one  body  to  another. 

Caroline,  Yet  surely,  Mrs.  B.,  there  are  other  pro- 
perties which  are  essential  to  bodies,  besides  those  you 
have  enumerated.  Colour  and  weight,  for  instance,  are 
common  to  all  bodies,  and  do  not  arise  from  their  con-» 
nection  with  each  other,  but  exist  in  the  bodies  them- 
selves ;  these,  therefore,  cannot  be  accidental  qualities  ? 

Mrs,  B,  I  beg  your  pardon ;  these  properties  do  not 
exist  in  bodies  independently  of  their  connection  with 
other  bodies. 

Caroline,  What !  have  bodies  no  weight  ?  Does  not 
this  table  weigh  heavier  than  this  book;  and,  if  one 
thing  weighs  heavier  than  another,  must  there  not  be 
such  a  thing  as  weight  ? 

Mrs,  B,  No  doubt:  but  this  property  does  not  ap- 
pear to  be  essential  to  bodies;  it  depends  upon  their 
connection  with  each  other.     Weight  is  an  effect  of  the 


ON  THE  ATTRACTION  OF  GRAVITY.  35 

power  of  attraction,  without  which  the  table  and  the 
book  would  have  no  weij^ht  whatever. 

Emily.  I  think  I  understand  jou  ;  is  it  not  the  at- 
traction of  gravity,  which  makes  bodies  heavy  ? 

Mrs,  B.  You  are  right.  I  told  v  u  that  the  attrac- 
tion of  gravity  was  proportioned  to  the  quantity  of  mat- 
ter which  bodies  contained :  now  the  earth  consisting 
of  a  much  greater  quantity  of  matter  than  any  body 
upon  its  surface,  the  force  of  its  attraction  must  neces- 
sarily be  greatest,  and  must  draw  every  thing  towards 
it ;  in  consequence  of  which,  bodies  that  are  unsupport- 
ed fall  to  the  ground,  whilst  those  that  are  supported 
press  upon  the  object  which  prevents  their  fall,  with  a 
weight  equal  to  the  force  with  which  they  gravitate  to- 
wards the  earth. 

Caroline.  The  same  cause  then  which  occasions  the 
fall  of  bodies,  produces  also  their  weight.  It  was  very 
dull  in  me  not  to  understand  this  before,  as  it  is  the 
natural  and  necessary  consequence  of  attraction  ;  but 
the  idea  that  bodies  were  not  really  heavy  of  themselves, 
appear  to  me  quite  incomprehensible.  But,  Mrs.  B.,  if 
attraction  is  a  property  essential  to  matter,  weight  must 
be  so  likewise ;  for  how  can  one  exist  without  the 
other  ? 

Mrs.  B.  Suppose  there  were  but  one  body  existing 
in  universal  space,  what  would  its  weight  be  ? 

Caroline.  That  would  depend  upon  its  size  ;  or, 
more  accurately  speaking,  upon  the  quantity  of  matter 
it  contained. 

Emily,  No,  no;  the  body  would  have  no  weight 
whatever  were  its  size ;  because  nothing  would  attract 
it.     Am  I  not  right,  Mrs.  B.  .^ 


36  GN  THE  ATTRACTION  OP  GRAVITY. 

Mrs.  B,  You  are  :  you  must  allow,  therefore,  that 
it  would  be  possible  for  attraction  to  exist  without 
weight ;  for  each  of  the  particles  of  which  the  body  was 
composed,  would  possess  the  power  of  attraction  ;  but 
they  could  exert  it  only  amongst  themselves ;  the  whole 
mass,  having  nothing  to  attract,  or  to  be  attracted  by, 
would  have  no  weight. 

Caroline,  1  am  now  well  satisfied  that  weight  is  not 
essential  to  the  existence  of  bodies  ;  but  what  have  you 
to  object  to  colours,  Mrs.  B. ;  you  will  not,  I  think,  deny 
that  they  really  exist  in  the  bodies  themselves. 

Mrs,  B,  When  we  come  to  treat  of  the  subject  of 
colours,  I  trust  that  I  shall  be  able  to  convince  you,  that 
colours  are  likewise  accidental  qualities,  quite  distinct 
from  the  bodies  to  which  they  appear  to  belong. 

Caroline,  Oh  do  pray  explain  it  to  us  now,  I  am  so 
rery  curious  to  know  how  that  is  possible. 

Mrs,  B,  Unless  we  proceed  with  some  degree  of  or- 
der and  method,  you  will  in  the  end  find  yourself  but 
little  the  wiser  for  all  you  learn.  Let  us  therefore  go 
©n  regularly,  and  make  ourselves  well  acquainted  with 
the  general  properties  of  bodies,  before  we  proceed 
further. 

Emily,  To  return,  then,  to  attraction,  (which  ap- 
pears to  me  by  far  the  most  interesting  of  them,  since 
it  belongs  equally  to  all  kinds  of  matter,)  it  must  be 
mutual  between  two  bodies ;  and  if  so,  when  a  stone 
falls  to  the  earth,  the  earth  should  rise  part  of  the  way 
to  meet  the  stone  ? 

Mrs,  B,  Certainly ;  but  you  must  recollect  that  the 
force  of  attraction  is  proportioned  to  the  quantity  of 
matter  which  bodies  contain,  and  if  ^ou  consider  the 


ON  THE  ATTRACTION  OF  GRAVITY.  3/ 

difference  there  is  in  that  respect,  between  a  stone  and 
the  earth,  you  will  not  be  surprised  that  you  do  not  per- 
ceive the  earth  rise  to  meet  the  stone;  for  though  it  is 
true  that  a  mutual  attraction  takes  place  between  the 
earth  and  the  stone,  that  of  the  latter  is  so  very  small  in 
comparison  to  that  of  the  former,  as  to  render  its  effect 
insensible. 

Emily.  But  since  attraction  is  proportioned  to  the 
quantity  of  matter  which  bodies  contain,  why  do  not 
the  hills  attract  the  houses  and  churches  towards  them? 

Caroline.  Heavens,  Emily,  what  an  idea!  How 
can  the  houses  and  churches  be  moved,  when  they  are 
io  firmly  fixed  in  the  ground  ? 

Mrs.  B.  Emily's  question  is  not  absurd,  and  your 
answer,  Caroline,  is  perfectly  just;  but  can  you  tell  us 
why  the  houses  and  churches  are  so  firmly  fixed  in  the 
ground  ? 

Caroline.  I  am  afraid  I  have  answered  right  by  mere 
chance ;  for  I  begin  to  suspect  that  bricklayers  and 
carpenters  could  give  but  little  stability  to  their  build- 
ings, without  the  aid  of  attraction. 

Mrs.  B.  It  is  certainly  the  cohesive  attraction  be- 
tween the  bricks  and  the  mortar,  which  enables  them  to 
build  walls,  and  these  are  so  strongly  attracted  by  the 
earth,  as  to  resist  every  other  impulse ;  otlierwise  they 
would  necessarily  move  towards  the  hills  and  the  moun- 
tains; but  the  lesser  force  must  yield  to  the  greater. 
There  are,  however,  some  circumstances  in  which  *he  at- 
traction of  a  large  body  has  sensibly  counteracted  that 
of  the  earth.  If,  whilst  standing  on  the  declivity  of  a 
mountain,  you  hold  a  plumb-line  in  your  hand,  the 
weight  will  not  fall  perpendicular  to  the  earth,  but  in- 


38  ON^  THE  ATTRACTION  OF  GRAVITY. 

dine  a  little  towards  the  mountain  ;  and  this  is  owing 
to  the  lateial,  or  sideways  attraction  of  the  mountain, 
interfering  with  the  perpendicular  attraction  of  the 
earth, 

Emily,  But  the  size  of  a  moujfitain  is  very  trifling 
compared  to  the  whole  earth  ? 

Mrs,  B.  Attraction,  you  must  recollect,  diminishes 
with  distance;  and  in  the  example  of  the  plumb-line, 
the  weight  suspended  is  considerably  nearer  to  the 
mountain  than  to  the  centre  of  the  earth. 

Caroline,  Pray,  Mrs.  B.,  do  the  two  scales  of  a  ba- 
lance hang  parallel  to  each  other  r 

Mrs,  B,  You  mean,  I  suppose,  in  other  words  to  in- 
quire whether  two  lines  which  are  perpendicular  to  the 
earth,  are  parallel  to  each  other  ?  I  believe  I  guess  the 
reason  of  your  question  ;  but  I  wish  you  would  endea- 
vour to  answer  it  without  my  assistance. 

Caroline,  I  was  thinking  that  such  lines  must  both 
tend  by  gravity  to  the  same  point,  the  centre  of  the  earth; 
now  lines  tending  to  the  same  point  cannot  be  parallel, 
as  parallel  lines  are  always  at  an  equal  distance  from 
each  other,  and  would  never  meet. 

Mrs.  B,  Yery  well  explained  ;  you  see  now  the  use 
of  your  knowledge  of  parallel  lines:  had  you  been  ig- 
noraut  of  their  properties,  you  could  not  have  drawn 
such  a  conclusion.  This  may  enable  you  to  form  an 
idea  of  the  great  advantage  to  be  derived  even  from  a 
slight  k?iowledge  of  geometry,  in  the  study  of  natural 
philosophy;  and  if,  after  1  have  made  you  acquainted 
with  the  first  elements,  you  should  be  tempted  to  pur- 
sue the  study,  I  would  advise  you  to  prepare  yourselves 
by  acquiring  some  knowledge  of  geometry.     This  sci- 


l^uh.  1>yU.YJIuJiq)7nrys  rtul^iJ? 


ON  THE  ATTRACTION  OF  GRAVITY.  39 

ence  would  teach  you  that  lines  which  fall  perpendicular 
to  the  surface  of  a  sphere  cannot  be  parallel,  because 
they  would  all  meet,  if  prolonged  to  the  centre  of  the 
sphere  ;  while  lines  that  fall  perpendicular  to  a  plane  or 
flat  surface,  are  always  parallel,  because  if  prolonged, 
they  would  never  meet. 

Emily.  And  y^t  a  pair  of  scales,  hanging  perpen-r 
dicular  to  the  earth,  appear  parallel  ? 

Mrs,  B.  Because  the  sphere  is  so  large,  and  the 
scales  consequently  converge  so  little,  that  their  incli- 
nation is  not  perceptible  to  our  senses  ;  if  we  could 
construct  a  pair  of  scales  whose  beam  would  extend 
several  degrees,  their  convergence  would  be  very  obvi- 
ous ;  but  as  this  cannot  be  accomplished,  let  us  draw  a. 
small  figure  of  the  earth,  and  then  we  may  make  a  pair 
of  scales  of  the  proportion  we  please,     (fig.  1.  plate  I.) 

Caroline.  This  figure  renders  it  very  clear :  then 
two  bodies  cannot  fall  to  the  earth  in  parallel  lines  ? 

Mrs.  B.    Never.' 

Caroline.  The  reason  that  a  heavy  body  falls  quick- 
er than  a  light  one,  is,  I  suppose,  because  the  earth  at- 
tracts it  more  strongly  } 

Mrs.  B.  The  earth,  it  is  true,  attracts  a  heavy  body 
more  than  a  light  one;  but  that  would  not  make  throne 
fall  quicker  than  the  other. 

Caroline.  Yet,  since  it  is  attraction  that  occasions 
the  fall  of  bodies,  surely  the  more  a  body  is  attracted, 
the  more  rapidly  it  will  fall.  Besides,  experience, 
proves  it  to  be  so.  Do  we  not  evety  day  see  heavy 
i)odies  fall  quickly,  and  light  bodies  slowly. 

Emily.  It  strikes  me,  as  it  does  Caroline,  that  as  at? 
traction  is  proportioned  to  the  quantity  of  matter,  the 


40  ON  THE  ATl'RAC  DION  OF  GRAVITY. 

earth  must  necessarily  attract  a  body  which  contains  a 
great  quantity  more  strongly,  and  therefore  bring  it  to 
the  ground  sooner  than  one  consisting  of  a  smaller 
quantity. 

Mrs,  B.  You  must  consider,  that  if  heavy  bodies  are 
attracted  more  strongly  than  light  ones,  they  require 
more  attraction  to  make  them  fall.  Remember  that 
bodies  have  no  natural  tendency  to  fall,  any  more  than 
to  rise,  or  to  move  laterally,  and  that  they  will  not  fall 
unless  impelled  by  some  force  ;  now  this  force  must  be 
proportioned  to  the  quantity  of  matter  it  has  to  move  : 
a  body  consisting  of  1000  particles  of  matter,  for  in- 
stance, requires  ten  times  as  much  attraction  to  bring  it 
to  the  ground  in  the  same  space  of  time  as  a  body  con- 
sisting of  only  100  particles. 

Caroline.  I  do  not  understand  that ;  for  it  seems  to 
mi%  that  the  heavier  a  body  is,  the  more  easily  and  rea- 
dily it  fails. 

Emily.  I  think  I  now  comprehend  it ;  let  me  try  if 
1  can  explain  it  to  Caroline.  Suppose  that  I  draw  to- 
wards me  two  weighty  bodies,  the  one  of  lOOlbs,  the 
other  of  lOOOlbs.,  must  I  not  exert  ten  times  as  much 
strength  to  draw  the  larger  one  to  me,  in  the  same  space 
of  time  as  is  required  for  the  smaller  one  ?  And  if  the 
earth  draws  a  body  of  lOOOlbs,  weight  to  it  in  the  same 
space  of  time  that  it  draws  a  body  of  lOOlbs,  does  it 
not  follow  that  it  attracts  the  body  of  lOOOlbs.  weight 
with  ten  times  the  force  that  it  does  that  of  lOOlbs.  ? 

Caroline.  I  comprehend  your  reasoning  perfectly ; 
but  if  it  were  so,  the  body  of  lOOOlbs.  weight,  and  that 
of  lOOlbs.  would  fall  with  the  same  rapidity;  and  the 
consequence  would  be,  that  all  bodies,  whether  light  or 
heavy,  being  at  an  equal  distance  from  the  ground, 


ON  THE  AITRACTION  OF  GRAVITY.  41 

would  fall  to  it  in  the  same  space  of  time :  now  it  is 
very  evident  that  this  conclusion  is  absurd  ;  experience 
every  instant  contradicts  it :  observe  how  much  sooner 
this  book  reaches  the  floor  than  this  sheet  of  paper, 
when  I  let  them  drop  together. 

Emily,  That  is  an  objection  I  cannot  answer.  I 
must  refer  it  to  you  Mrs.  B. 

Mrs,  B.  1  trust  that  we  -shall  not  find  it  insurmount- 
able. It  is  true  that,  according  to  the  laws  of  attraction, 
all  bodies  at  an  equal  distance  from  the  earth,  should 
fall  to  it  in  the  same  space  of  time  ;  and  this  would  ac* 
tually  take  place  if  no  obstacle  intervened  to  impede 
their  fall.  But  bodies  falls  through  the  air,  and  it  is 
the  resistance  of  the  air  which  makes  bodies  of  different 
density  fall  with  different  degrees  of  velocity.  They 
must  all  force  their  way  through  the  air,  but  dense 
heavy  bodies  overcome  this  obstacle  more  easily  than 
rarer  and  lighter  ones. 

The  resistance  which  the  air  opposes  to  the  fall  of 
bodies  is  proportioned  to  their  surface,  not  to  their 
weight;  the  air  being  inert,  cannot  exert  a  greater 
force  to  support  the  weight  of  a  cannon-bail,  than  it 
does  to  support  the  weight  of  a  ball  (of  the  same  size) 
made  of  leather;  but  the  cannon-ball  will  overcome 
this  resistance  more  easily,  and  fall  to  the  ground,  con- 
sequently, quicker  than  the  leather  ball. 

Caroline,  This  is  very  clear,  and  solves  the  difficul-^ 
ty  perfectly.  The  air  offers  the  same  resistance  to  a 
bit  of  lead  and  a  bit  of  feather  of  the  same  size  ;  yet 
the  one  seems  to  meet  with  no  obstruction  in  its  fall, 
whilst  the  odier  is  evidently  resisted  and  supported  for 
some  time  by  the  air. 
D  2 


42  ON  THE  ATTRACriON  OF  GRAVITY: 

Emily.  The  larger  the  surface  of  a  body,  then,  th^ 
mor«  air  it  covers,  and  the  greater  is  the  resistance  it 
meets  with  from  it. 

Mrs,  B,  Certainly  :  observe  the  manner  in  which 
this  sheet  of  paper  falls  ;  it  floats  awhile  in  the  air,  and 
then  gently  descends  to  the  ground.  I  will  roll  the  same 
piece  of  paper  up  into  a  ball :  it  offers  now  but  a  small 
surface  to  the  air,  and  encounters  therefore  but  little 
resistance :  see  how  much  more  rapidly  it  falls. 

The  heaviest  bodies  may  be  made  to  float  awhile  iri 
the  air,  by  making  the  extent  of  their  surface  counter- 
balance their  weight.  Here  is  some  gold,  which  is  the 
most  dense  body  we  are  acquainted  with,  but  it  has 
been  beaten  into  a  very  thin  leaf,  and  offers  so  great  an 
extent  of  surface  in  proportion  to  its  weight,  that  its 
fall,  you  see,  is  still  more  retarded  by  the  resistance  of 
the  air  than  that  of  the  sheet  of  paper. 

Caroline,  That  is  very  curious ;  and  it  is,  I  suppose, 
upon  the  same  principle  that  iron  boats  may  be  made  to 
float  on  water  ? 

But,  Mrs.  B..  if  the  air  is  a  real  body,  is  it  not  also  sub- 
jected to  the  laws  of  gravity  ? 

Mrs.  B,    Undoubtedly. 

Caroline,  Then  why  does  it  not,  like  all  other  bodies, 
fall  to  the  ground  ? 

Mrs,  B,  On  account  of  its  spring  or  elasticit3\  The 
air  is  an  elastic  fluid  ;  a  species  of  bodies,  the  peculiar 
property  of  which  is  to  resume,  after  compression,  their 
original  dimensions  ;  and  you  must  consider  the  air  of 
which  the  atmosphere  is  composed  as  existing  in  a  state 
of  compression,  for  its  particles  being  drawn  towards  the 
earth  by  gravity,  are  brought  closer  together  than  they 


ON  THE  ATTRACTION  OP  GRAVITY.  43 

would  otherwise  be,  but  the  spring  or  elasticity  of  the  aip 
by  which  it  endeavours  to  resist  compression  gives  it  a 
Constant  tendency  to  expand  itself,  so  as  to  resume  the 
dimensions  it  would  naturally  have,  if  not  under  the  in- 
fluence of  gravity.  The  air  may  therefore  be  said  con- 
stantly to  struggle  with  the  power  of  gravity  without 
being  able  to  overcome  it.  Gravity  thus  confines  the  air 
to  the  regions  of  our  globe,  whilst  its  elasticity  prevents 
it  from  falling  like  other  bodies  to  the  ground. 

Emily,  The  air  then  is  I  suppose,  thicker,  or  I  should 
rather  say  more  dense,  near  the  surface  of  the  earth, 
than  in  the  higher  regions  of  the  atmosphere  ;  for  that 
part  of  the  air  which  is  nearer  the  surface  of  the  earth 
must  be  most  strongly  attracted. 

Mrs.  B,  The  diminution  of  the  force  of  gravity,  at  so 
small  a  distance  as  that  to  which  the  atmosphere  extends 
(compared  with  the  size  of  the  earth)  is  so  inconsiderable 
as  !0  be  scarcely  sensible ;  but  the  pressure  of  the  upper 
parts  of  the  atmosphere  on  those  beneath,  renders  the 
air  near  the  surface  of  the  earth  much  more  dense  than 
the  upper  regions.  The  pressure  of  the  atmosphere  has 
been  compared  to  that  of  a  pile  of  fleeces  of  wool,  in 
which  the  lower  fleeces  are  pressed  together  b>  the 
weight  of  those  above  ;  these  lie  light  and  loose,  in  pro- 
portion as  they  approach  the  uppermost  fleece,  which  re- 
ceives no  external  pressure,  and  is  confined  merely  by 
the  force  of  its  own  gravity. 

Caroline.  It  has  just  occurred  to  me  that  there  are 
some  bodies  which  do  not  gravitate  towards  the  earth. 
Smoke  and  steam,  for  instance,  rise  instead  of  falling. 

Mrs.  B,  It  is  still  gravity  which  produces  their  as- 
cent ;  at  least,  were  that  power  destroyed,  these  bodies 
would  not  risev 


44  ON  THE  ATTRACTION  OF  GRAVITY 

Caroline.  I  shall  be  out  of  conceit  witli  gravity,  if  it 
is  so  inconsistent  in  its  operations. 

Mrs,  B.  There  is  no  difficulty  in  reconciling  this 
apparent  inconsistency  of  effect.  The  air  near  the 
earth  is  heavier  than  smoke,  steam  or  other  vapours  ;  it 
consequently  not  only  supports  these  light  bodies,  but 
forces  them  to  rise,  till  they  reach  a  part  of  the  atmos- 
phere, the  weight  of  which  is  not  greater  than  their  own, 
and  then  they  remain  stationar3^  Look  at  this  bason 
of  water ;  why  does  the  piece  of  paper  which  I  throw 
into  it  float  on  the  surface  ? 

Emily,  Because,  being  lighter  than  the  water,  it  is 
supported  by  it. 

Mrs,  B,  And  how  that  I  pour  more  water  into  the 
bason,  why  does  the  paper  rise? 

Emily,  The  water  being  heavier  than  the  paper,  gets 
beneath  it,  and  obliges  it  to  rise. 

Mrs,  B,  In  a  similar  manner  are  smoke  and  vapour 
forced  upwards  by  the  air;  but  these  bodies  do  not, 
like  the  paper  ascend  to  the  surface  of  the  fluid,  be- 
cause, as  we  observed  before,  the  air  being  thinner  and 
lighter  as  it  is  more  distant  from  the  earth,  vapours  rise 
only  till  they  attain  a  region  of  air  of  their  own  den- 
sity. Smoke,  indeed,  ascends  but  a  \try  little  way  ;  it 
consists  of  minute  particles  of  fuel  carried  up  by  a  cur- 
rent of  heated  air  from  the  fire  below  :.  heat  you  recol- 
lect, expands  all  bodies ;  it  consequently  rarefies  air, 
and  renders  it  lighter  than  the  colder  air  of  the  atmos- 
phere; the  heated  air  from  the  tire  carries  up  with  it  va- 
pour and  small  particles  of  the  combustible  materials 
which  are  burning  in  the  fire.  When  this  curr'ent  of 
hot  air  is  cooled  by  "mixing  with  that  of  the  atmosphere 


ON  TH£  ATTRACTION  OF  GRAVITY.  45 

tbe  minute  particles  of  coal  or  other  combustible  fall, 
it  is  this  which  produces  the  small  blacK  flakes  which 
render  the  air  and  every  thing  in  contact  with  it,  in 
London,  so  dirty. 

Caroline.  You  must,  however,  allow  me  to  make  one 
more  objection  to  the  universal  gravity  of  bodies  :  which 
is  the  ascent  of  air-balloons,  the  materials  of  which  are 
undoubtedly  heavier  ihan  air :  how,  therefore,  can  they 
be  supported  by  it  ? 

Mrs.  B.  I  admit  that  the  materials  of  which  bal- 
loons are  made  are  heavier  than  the  air  ;  but  the  air  with 
which  they  are  filled  is  an  elastic  fluid,  of  a  different 
nature  from  the  atmospheric  air,  and  considerably  light- 
er ;  so  that  on  the  whole,  the  balloon  is  lighter  than  the 
air  which  it  displaces,  and  consequently  will  rise,  on 
the  same  principle  as  smoke  and  vapour.  Now  Emily 
let  me  hear  if  you  <:an  explain  how  the  gravity  of  bodies 
is  modified  by  the  effect  of  the  air? 

Emily,  The  air  forces  bodies  which  are  lighter  than 
itself  to  ascend  ;  those  that  are  of  an  equal  weight  will 
remain  stationary  in  it;  and  those  that  are  heavier  will 
descend  through  it :  but  the  air  will  have  some  effect  on 
these  last ;  for  if  they  are  not  much  heavier,  they  will 
witii  difficulty  overcome  the  resistance  they  meet  with 
in  passing  through  it,  they  will  be  borne  up  by  it,  and 
their  fall  will  be  more  or  less  retarded. 

Mrs.  B.  Very  well.  Observe  how  slowly  this  light 
feather  falls  to  the  ground,  while  a  heavier  body,  like  this 
marble,  overcomes  the  resistance  which  the  air  makes 
t-o  its  descent  much  more  easily,  and  its  fall  is  propor- 
tionally more  rapid.  I  now  throw  a  pebble  into  this  tub 
of  water ;  it  does  not  reach  the  bottom  near  so  soon  as  if 


46  ON  THE  ATTRACTION  OF  GRAVITY. 

there  were  no  water  in  the  tub,  because  it  meets  with 
resistance  from  the  water.  Suppose  that  we  could  emp- 
ty the  tub,  not  only  of  water,  but  of  air  also,  the  peb- 
ble would  then  fall  quicker  still,  as  it  would  in  that 
case  meet  with  no  resistance  at  all  to  counteract  its 
gravity. 

Thus  you  see  that  it  is  not  the  different  degrees  of 
gravity,  but  the  resistance  of  the  air,  which,  prevents 
bodies  of  different  weight  from  falling  with  equal  velo- 
cities;  if  the  air  did  not  bear  up  the  feather,  it  would 
reach  the  ground  as  soon  as  the  marble. 

Caroline,     I  make  no  doubt  that  it   is  so;  and  yet 

1  do  not  feel  quite  satisfied.  I  wish  there  was  any  place 
void  of  air,  in  which  the  experiment  could  be  made. 

Mrs.  B.  If  that  proof  will  satisfy  your  doubts,  I  can 
give  it  you.  Here  is  a  machine  called  an  air  pump,  (fig. 
2.  pi.  J.)  by  means  of  which  the  air  may  be  expelled 
from  any  close  vessel  which  is  placed  over  this  opening, 
through  which  the  air  is  pumped  out.  Glasses  of  vari- 
ous shapes,  usually  called  receivers,  are  employed  for 
this  purpose.  We  shall  now  exhaust  the  air  from  this 
tall  receiver  which  is  placed  over  the  opening,  and  we 
shall  find  that  bodies  of  whatever  weight  or  size  within 
it,  will  fall  from  the  top  to  the  bottom  in  the  same  space 
of  time. 

Caroline,  Oh,  I  shall  be  delighted  with  this  experi- 
ment ;  what  a  curious  machine  !  how  can  you  put  the 
two  bodies  of  different  weight  within  the  glass,  with- 
out admitting  the  air. 

Mrs.  B.  A  guinea  and  a  feather  are  already  placed 
there  for  the  purpose  of  the  experiment :  here  is  you  see 

2  contrivance  to  fasten  them  in  the  upper  part  of  the 


ON  THE  ATTRACTION  OP  GRAVITY.  47 

glass;  as  soon  as  the  air  is  pumped  out,  I  shall  turn  this 
little  screw,  by  which  means  the  brass  plates  which 
support  them  will  be  inclined,  and  the  two  bodies  will 
fall. — Now  I  believe  I  have  pretty  well  exhausted  the 
air. 

Caroline.  Pray  let  me  turn  the  screw, — I  declare, 
they  both  reached  the  bottom  at  the  same  instant !  Did 
you  see,  Emily,  the  feather  appeared  as  heavy  as  the 
guinea? 

Emily,    Exactly  ;  and  fell  just  as  quickly.     How 

wonderful  this  is !  what  a  number  of  entertaining  ex- 
es 

periments  might  be  made  with  this  machine  ! 

Mrs.  B,  No  doubt  there  are  a  great  variety  ;  but  we 
shall  reserve  them  to  elucidate  the  subjects  to  which  they 
relate :  if  I  had  not  explained  to  you  why  the  guinea 
and  the  feather  fell  with  equal  velocity,  you  would 
not   have  been  so  well  pleased  with  the  experiment. 

Emily.  I  should  have  been  as  much  surprised,  but 
not  so  much  interested  ;  besides  experiments  help  to 
imprint  on  the  memory  the  facts  they  are  intended  to 
illustrate  ;  it  will  be  better  therefore  for  us  to  retain  our 
curiosity,  and  wait  for  other  experiments  in  their  pro- 
per places. 

Caroline.  Pray  by  what  means  is  the  air  exhausted 
in  this  receiver  ? 

Mrs.  B.  You  must  learn  something  of  mechanics 
in  order  to  understand  the  construction  of  a  pump.  At 
our  next  meeting,  therefore,  I  shall  endeavour  to  Miake 
you  acquainted  with  the  laws  of  motion,  as  an  intro- 
duction to  that  subject. 


CONVEttSATION  HI, 


ON  THE  LAWS  OF  MOTION. 

Oi^  MOTION.— or  THE  INERTIA  OP  BODIES.— OF  FORCE  TO  PRODUGX 
MOTION.— DIRECTION  OF  MOTION. — VELOCITY,  ABSOLUTE  AND 
RELATIVE.— UNIFORM  MOTION.— RETARDEl^  MOTION. — ACCELE- 
RATED  MOTION. VELOCITY   OF   FALLINO    BODIES. — MOMENTUM. 

—ACTION  AND  REACTION  EaUAL.  -ELASTICITY  OF  BODIES.— 
POROS^ITY  OF  BODIES.— REFLICIED  MOTION. ANGLES  OF  INCI- 
DENCE AND  REFLECTION. 


Mrs.  B. 

The  science  of  mechanics  is  founded  on  the  laws  of 
motion ;  it  wiJI  therefore  be  necessary  to  make  you  ac- 
quainted with  these  laws  before  we  examine  the  me- 
chanical powers.  Tell  me,  Caroline,  what  do  you  un- 
derstand bv  the  word  motion  ? 

Caroline.  I  think  I  understand  it  perfectly,  though 
I  am  at  a  loss  to  describe  it.  Motion  is  the  act  of  mov- 
ing; about,  going  from  one  place  to  another,  it  is  the  con- 
trary of  remaining  at  rest. 

E 


aU  ON  THE  LAWS  OF  MOTION. 

Mrs,  B.  Very  well.  Motion  then  consists  in  a 
change  of  place ;  a  body  is  in  motion  whenever  it  is 
changing  its  situation  with  regard  to  a  fixed  point. 

Now  since  we  have  observed  that  one  of  the  general 
properties  of  bodies  is  Inertia,  that  is,  an  entire  pas- 
siveness  either  with  regard  to  motion  or  rest,  it  follows 
that  a  body  cannot  move  without  being  put  into  motion; 
the  power  which  puts  a  body  into  motion  is  called /orce  ; 
thus  the  stroke  of  the  hammer  is  the  force  which  drives 
the  nail ;  the  pulling  of  the  horse  that  which  draws  the 
carriage,  &.c.  Force  then  is  the  cause  which  produces 
motion. 

Emily.  And  may  we  not  say  that  gravity  is  the 
force  which  occasions  the  fall  of  bodies  ? 

Mrs,  B.  Undoubtedly.  I  had  given  you  the  most 
familiar  illustrations  in  order  to  render  the  explanation 
clear;  but  since  you  seek  for  more  scientific  examples, 
you  may  say  that  cohesion  is  the  force  which  binds  the 
particles  of  bodies  together,  and  heat  that  which  drives 
them  asunder. 

The  motion  of  a  body  acted  upon  by  a  single  force  is 
always  in  a  straight  line,  in  the  direction  in  which  it  re- 
ceived the  impulse. 

Caroline.  That  is  very  natural ;  for  as  the  body  is 
inert,  and  can  move  only  because  it  is  impelled,  it  will 
move  only  in  the  direction  in  which  it  is  impelled.  The 
degree  of  quickness  with  which  it  moves,  must  I  sup- 
pose, also  depend  upon  the  degree  of  force  with  which 
it  is  impelled. 

Mrs,  B,  Yes ;  the  rate  at  which  a  body  moves,  or 
the  shortness  of  the  time  which  it  takes  to  move  from 
one  place  to  another,  is  called  its  velocity  ;  and  it  is 


ON  THE  LAWS  OF  MOTION.  5i 

one  of  the  laws  of  motion  that  the  velocity  of  the  mov- 
ing body  is  proportional  to  the  force  by  which  it  is  put 
in  motion.  We  must  distinguish  between  absolute  and 
relative  velocity. 

The  velocity  of  a  body  is  called  absolute,  if  we  con- 
sider the  motion  of  the  body  in  space,  without  any  re- 
ference to  that  of  other  bodies.  When  for  instance  a 
horse  goes  fifty  miles  in  ten  hours,  his  velocity  is  five 
miles  an  hour. 

The  velocity  of  a  body  is  termed  relative,  when  coTa- 
pared  with  that  of  another  body  which  is  itself  in  mo- 
tion. For  instance,  if  one  man  walks  at  the  rate  of  a 
mile  an  hour,  and  another  at  the  rate  of  two  miles  an 
hour,  the  relative  velocity  of  the  latter  is  double  that  of 
the  former ;  but  the  absolute  velocity  of  the  one  is  one 
mile,  and  that  of  the  other  two  miles  an  hour. 

Emilif.  Let  me  see  if  I  understand  it — The  relative 
velocity  of  a  body  is  the  degree  of  rapidity  of  its  mo- 
tion compared  with  that  of  another  body ;  thus  if  one 
ship  sail  three  times  as  far  as  another  ship  in  the  same 
space  of  time,  the  velocity  of  tiie  former  is  equal  to 
three  times  that  of  the  latter. 

Mrs.  B,  The  general  rule  may  be  expressed  thus  : 
the  velocity  of  a  body  is  measured  by  the  space  over 
which  it  moves,  divided  by  the  time  which  it  employs 
in  that  motion :  thus  if  you  travel  one  hundred  miles 
in  twenty  hours,  what  is  your  velocity  in  each  hour  ? 

Emily.  I  must  divide  the  space,  which  is  one  hun- 
dred miles,  by  th«  time,  which  is  twenty  hours,  and  the 
answer  will  be  five  milfts  an  hour.  Then,  Mrs.  B.,  may 
we  not  reverse  this  rule  and  say,  that  the  time  is  equal 
t<3  the  space  divided  by  the  velocity ;  since  the  space 


52  ON  THE  LAWS  OF  MOTION. 

one  hundred  miles,  divided  by  the  velocity  five  miles, 
gives  twenty  hours  for  the  time  ? 

J\Irs,  B.  Certainly ;  and  we  may  say  also  that  space 
is  equal  to  the  velocity  multiplied  by  the  time.  Can 
you  tell  me,  Caroline,  how  many  miles  you  will  have 
travelled,  if  your  velocity  is  three  miles  an  hour  and 
you  travel  six  hours  ? 

Caroline,  Eighteen  miles ;  for  the  product  of  -3 
multiplied  by  6,  is  18. 

Mrs,  B.  I  suppose  that  you  understand  what  is 
meant  by  the  terms  uniform,  accelerated  and  retarded 
motion. 

Emily,  I  conceive  uniform  motion  to  be  that  of  a  body 
whose  motion  is  regular,  and  at  an  equal  rate  through- 
out ;  for  instance,  a  horse  that  goes  an  equal  number  of 
miles  every  hour.  But  the  hand  of  a  watch  is  a  much 
better  example,  as  its  motion  is  so  regular  as  to  indicate 
the  time. 

Mrs,  B,  You  have  a  right  idea  of  uniform  motion  ; 
but  it  would  be  more  correctly  expressed  by  saying,  that 
the  motion  of  a  body  is  uniform  when  it  passes  over 
equal  spaces  in  equal  times.  Uniform  motion  is  pro- 
duced by  a  force  having  acted  on  a  body  once,  and  hav- 
ing ceased  to  act;  as  for  instance,  the  stroke  of  a  bat 
oil  a  cricket  ball. 

Caroline.  But  the  motion  of  a  cricket  ball  is  not  uni- 
form ;  its  velocity  gradually  diminishes  till  it  falls  to  the 
ground. 

Mrs.  B.  Recollect  that  the  cricket,  ball  is  inert,  and 
has  no  more  power  to  stop  than  to  put  itself  in  motion  ; 
if  it  falls,  therefore,  it  must  be  stopped  by  some  force 
superior  to  that  by  which  it  was  projected,  and  which 
destroys  its  motion^ 


ON  THE  LAWS  OF  MOTION.  53 

Caroline.  And  it  is  no  doubt  the  force  of  gravity 
which  counteracts  and  destroys  that  of  projection  ;  but 
if  there  were  no  such  power  as  gravity,  would  the 
cricket  ball  never  stop  ? 

Mrs.  B.  If  neither  gravity  nor  any  other  force, 
such  as  the  resistance  of  the  air,  opposed  its  motion,  tlie 
cricket  ball,  or  even  a  stone  thrown  by  the  hand,  would 
proceed  onwards  in  a  right  line,  and  with  an  uniform 
velocity  for  ever. 

Caroline,  You  astonish  me !  1  thought  that  it  was 
impossible  to  produce  perpetual  motion  ? 

Mrs.  B.  Perpetual  motion  cannot  be  produced  by 
art,  because  gravity  ultimately  destroys  all  motion  that 
human  powers  can  produce. 

Emily^  But  independently  of  the  force  of  gravity^ 
I  cannot  conceive  that  the  little  motion  I  am  capable  of 
giving  to  a  stone  would  put  it  in  motion  for  ever. 

Mrs.  B,  The  quantity  of  motion  you  communicate 
to  the  stone  would  not  influence  its  duration;  if  you. 
threw  it  with  little  force  it  would  move  slowly,  for  its 
velocity,  you  must  remember,  will  be  proportional  to 
the  force  with  which  it  is  projected ;  but  if  there  is  no- 
thing to  obstruct  its  passage,  it  will  continue  to  move 
with  the  same  velocity,  and  in  the  same  direction  as 
when  you  first  projected  it. 

Caroline.  This  appears  to  me  quite  incomprehensi- 
ble ;  we  do  not  meet  with  a  single  instance  of  it  in 
nature. 

Mr^.  B,     I  beg  your  pardon.     When  you  come  to 

stud  V  the  m{>tion  of  the  celestial  bodies,  you  will  find 

that  nature  abourjds  with  examples  of  perpetual  motion  ; 

and  that  it  conduces  as  much  to  the  harmony  of  the 

e2 


U  ON  THE  LAWS  OP  MOTION. 

system  of  the  universe,  as  the  prevalence  of  it  would 
to  the  destruction  of  all  comfort  on  our  globe.  The 
wisdom  of  Providence  has  therefore  ordained  insur- 
mountable obstacles  to  perpetual  motion  here  below, 
and  though  these  obstacles  often  compel  us  to  contend 
with  great  difficulties,  yet  there  results  from  it  that  or- 
der, regularity  and  repose,  so  essential  to  the  preserva- 
tion of  all  the  various  beings  of  which  this  world  is 
composed. 

Now  can  you  tell  me  what  is  retarded  motion  ? 
Caroline.  Retarded  motion  is  that  of  a  body  which 
moves  every  moment  slower  and  slower  :  thus  when  I 
am  tired  with  walking  fast,  I  slacken  my  pace ;  or  when 
a  stone  is  thrown  upwards,  its  velocity  is  gradually  di- 
minished by  the  power  of  gravity. 

Mrs,  B.  Retarded  motion  is  produced  by  some 
force  acting  upon  the  body  in  a  direction  opposite  to  that 
which  first  put  it  in  motion  :  you  who  are  an  animated 
being,  endowed  with  power  and  will,  may  slacken  your 
pace,  or  stop  to  rest  when  you  are  tired ;  but  inert  mat- 
ter is  incapable  of  any  feeling  of  fatigue,  can  never 
slacken  its  pace,  and  never  stop,  unless  retarded  or  ar- 
rested in  its  course  by  some  opposing  force  ;  and  as  it  is 
the  laws  of  inert  bodies  which  mechanics  treats  of,  I  pre- 
fer your  illustration  of  the  stone  retarded  in  its  ascent. 
Now  Emily,  it  is  your  turn ;  what  is  accelerated  motion? 
Emily.  Accelerated  motion,  I  suppose,  takes  place 
when  the  velocity  of  a  body  is  increased  ;  if  you  had 
not  objected  to  our  giving  such  active  bodies  as  our- 
selves as  examples,  I  should  say  that  my  motion  is  ac- 
celerated if  I  change  my  pace  from  walking  to  running. 
I  cannot  think  of  any  instance  of  accelerated  motion 


QN  THE  LAWS  OF  MOTION'.  5^ 

in  inanimate  bodies  ;  all  motion  of  inert  matter  seems 
to  be  retarded  by  gravity. 

Mm.  B,  Not  in  all  cases  ;  for  the  power  of  gravita- 
tion sometimes  produces  accelerated  motion;  for  in- 
stance, a  stone  falling  from  a  height  moves  with  a  re- 
gularly accelerated  motion. 

Emily.  True ;  because  the  nearer  it  approaches  the 
earth,  the  more  it  is  attracted  by  it. 

Mrs,  B.  You  have'  mistaken  the  cause  of  its  acce- 
leration of  motion  ;  for  though  it  is  true  that  the  force 
of  gravity  increases  as  a  body  approaches  the  earth, 
the  difference  is  so  trifling  at  any  small  distance  froria 
its  surface  as  not  to  be  perceptible. 

Accelerated  motion  is  produced  when  the  force  which 
put  a  body  in  motion  continues  to  act  upon  it  during  its 
motion,  so  that  its  motion  is  continually  increased. 
When  a  stone  falls  from  a  height,  the  impulse  which  it 
receives  from  gravity  during  the  first  instant  of  its  fall, 
would  be  sufficient  to  bring  it  to  the  ground  with  a  uni- 
form velocity  :  for,  as  we  have  observed,  a  body  having 
been  once  acted  upon  by  a  force,  will  continue  to  move 
with  a  uniform  velocity;  but  the  stone  is  not  acted  upon 
by  gravity  merely  at  the  first  instant  of  its  fall,  this 
power  continues  to  impel  it  during  the  whole  of  its  de- 
scent, and  it  is  this  continued  impulse  which  accelerates 
its  motion. 

Emily.  I  do  not  quite  understand  that. 
^  Mrs,  B.  Let  us  suppose  that  the  instant  after  you 
have  let  fall  a  stone  from  a  high  tower,  the  force  of  gra- 
vity were  annihilated,  the  body  would  nevertheless  con- 
tinue to  move  downwards,  for  it  would  have  received  a 
first  impulse  from  gravity,  ^nd  a. body  once.put  in  mo- 


56  ON  THE  LAWS  OP  MOTION. 

tion  will  not  stop  unless  it  meets  with  some  obstacle  to 
impede  its  course ;  in  this  case  its  velocity  would  be 
uniform,  for  though  there  would  be  no  obstacle  to  ob- 
struct its  descent,  there  would  be  no  force  to  accelerate 
it. 

Emily.    That  is  very  clear. 

Mrs.  B,  Then  you  have  only  to  add  the  power  of 
gravity  constantly  acting  on  the  stone  during  its  descent, 
and  it  will  not  be  difficult  to  understand  that  its  motion 
will  become  accelerated,  since  the  gravity  which  acts 
on  the  stone  during  the  first  instant  of  its  descent,  will 
continue  in  force  every  instant  till  it  reaches  the  o;rou1id. 
Let  us  suppose  that  the  impulse  given  by  gravity  to  the 
stone  during  the  first  instant  of  its  descent  be  equal  to 
one,  the  next  instant  we  shall  find  that  an  additional 
impulse  gives  the  stone  an  additional  velocity  equal  to 
one,  so  that  the  accumulated  velocity  is  now  equal  to 
two;  the  following  instant  another  impulse  increases 
the  velocity  to  three,  and  so  on  till  the  stone  reaches 
the  ground. 

Caroline.  Now  I  understand  it;  the  effects  of  pre- 
ceding impulses  must  be  added  to  the  subsequent 
velocities. 

Mrs.  B.  Yes  ;  it  has  been  ascertained,  both  by  ex- 
periment and  calculations,  which  it  would  be  too  diffi- 
cult for  us  to  enter  into,  that  heavy  bodies  desce'^uling 
from  a  height  by  the  force  of  gravity,  fall  sixteen  teet 
the  first  second  of  time,  three  times  that  dis-ance  in 
the  next,  five  times  in  the  third  second,  seven  times  in 
the  fourth,  and  soon,  regularly  increasing  their  velo- 
cities according  to  the  nutnber  of  seconds  during 
which  the  body  has  been  falling. 


ON  THE  LAWS  OF  MOTION.  5,7 

Emily,  If  jou  throw  a  stone  perpendicularly  up- 
wards, is  it  not  the  same  length  of  time  ascending  that 
it  is  descending  ? 

Mrs,  B.  Exactly;  in  ascending,  the  velocity  is  di* 
minished  by  the  force  of  gravity ;  in  descending,  it  is 
accelerated  by  it. 

Caroline.  I  should  then  have  imagined  that  it  would 
have  fallen  quicker  than  it  rose  ? 

Mrs.  B.  You  must  recollect  that  the  force  with  which 
it  is  projected  must  be  taken  into  the  account ;  and  that 
this  force  is  overcome  and  destroyed  by  gravity  before 
the  body  falls. 

Caroline.  But  the  force  of  projection  given  to  a  stone 
in  throwing  it  upwards,  cannot  always  be  equal  to  the 
force  of  gravity  in  bringing  it  down  again,  for  the  force 
of  gravity  is  always  the  same,  whilst  the  degree  of  im- 
pulse given  to  the  stone  is  optional ;  I  may  throw  it  up 
gently,  or  with  violence. 

Mrs.  B.  If  you  throw  it  gently,  it  will  not  rise  high  ; 
perhaps  only  sixteen  feet,  in  which  case  it  will  fall  in 
©ne  second  of  time.  Now  it  is  proved  by  experiment, 
that  an  impulse  requisite  to  project  a  body  sixteen  feet 
upwards,  will  make  it  ascend  that  height  in  one  second  ; 
here  then  the  times  of  the  ascent  and  descent  are  equal. 
But  supposing  it  be  required  to  throw  a  stone  twice 
that  height,  the  force  must  be  proportionally  greater. 

Mrs.  B.  You  see  then,  that  the  impulse  of  projec- 
tion in  throwing  a  body  upwards,  is  always  equal  to  the 
action  of  the  force  of  gravity  during  its  descent ;  and 
that  it  is  the  greater  or  less  distance  to  which  the  body 
rises,  that  makes  these  two  forces  balance  each  other. 

I  must  now  expIaLi  'o  you  what  is  mecnt  by  the  wo- 
rrKUtum  of  bodies.    It  is  the  force,  or  power>  with  which 


58  ON  THE  LAW9  OF  MOTIOK, 

a  body  in  motion,  strikes  against  another  body.  The 
momentum  of  a  body  is  composed  of  its  qunntity  of 
matter,  multiplied  by  its  quantity  of  motion  ^  in  other 
words,  its  weight  and  its  velocity. 

Caroline*  The  quicker  a  body  moves,  the  greater, 
no  doubt,  must  be  the  force  with  which  it  would  strike 
against  another  body* 

Emily,  Therefore  a  small  body  may  have  a  greater 
momentum  than  a  large  one,  provided  its  velocity  be 
sufficiently  greater ;  for  instance,  the  momentum  of  an 
arrow  shot  from  a  bow,  must  be  greater  than  a  stone 
thrown  by  the  hand. 

Caroline.  We  know  also  by  experience,  that  the 
heavier  a  body  is,  the  greater  is  its  force ;  it  is  not 
therefore  difficult  to  understand,  that  the  whole  power 
©r  momentum  of  a  body  miist  be  composed  of  these  two 
properties  :  but  I  do  not  understand,  why  they  should 
be  multiplied,  the  one  by  the  other ;  I  should  have  sup- 
posed that  the  quantity  of  matter  should  have  been  ad^ 
ded  to  the  quantity  of  motion  ? 

Mrs,  B,  It  is  found  by  experiment,  that  if  the 
weight  of  a  body  is  represented  by  the  number  3,  and 
its  velocity  also  by  3,  its  momentum  will  be  represented' 
by  9 ;  not  6,  as  would  be  the  case,  were  these  figures 
added,  instead  of  being  multiplied  together.  I  recom- 
mend it  to  you  to  be  careful  to  remember  the  definition 
of  the  momentum  of  bod-es,  as  it  is  one  of  the  most  im- 
portant points  in  mechanics ;  you  will  find,  that  it  is 
from  opposing  motion  to  matter,  that  machines  derive 
their  powers*. 

*  In  comparing"  together  *^^he  momenta  of  different  bodies, 
we  must  be  attentive  to  measure  their  weig^lits  and  velocities; 


Ol^  THB  LAWS  05f  MOTIOK*.  59 

The  reaction  of  bodies,  is  the  next  law  of  motion 
which  I  must  explain  to  you.  When  a  body  in  motion 
strikes  against  another  body,  it  meets  with  resistance 
from  it ;  the  resistance  of  the  body  at  rest,  will  be  equal 
to  the  blow  struck  by  the  body  in  motion  ;  or  to  express 
myself  in  philosophical  language,  action  SLud  re-action 
will  be  equal,  and  in  opposite  directions. 

Caroline.  Do  you  mean  to  say,  that  the  action  of  tlie 
body  which  strikes,  is  returned  with  equal  force  by  the 
body  which  receives  the  blow. 

Mrs,  B.    Exactly. 

Caroline.  But  if  a  man  strikes  another  on  the  face 
with  his  fist,  he  surely  does  not  receive  as  much  pain  by 
the  re-action,  as  he  inflicts  by  the  blow  ? 

Mrs.  B.  No ;  but  this  is  simply  owing  to  the  knuc- 
kles having  much  less  feeling,  that  the  face. 

Here  are  two  ivory  balls  suspended  by  threads,  (plate 
I.  fiij;.  3.)  draw  one  of  them.  A,  a  little  on  one  side, — now 
let  it  go  ; — it  strikes,  you  see,  against  the  other  ball  B, 
and  drives  it  off,  to  a  distance  equal  to  that  through 
which  the  first  ball  fell  ;  but  the  motion  of  A  is  stopped, 
because  when  it  struck  B,  it  received  in  return  a  blow 
equal  to  tliat  it  gave,  and  its  motion  was  consequently 
destroyed. 


by  the  same  denomination  of  weights  and  of  spaces,  otherwise 
the  resiilts  would  not  agree.  Thus  if  we  estimate  the  weight  of 
one  body  m  ounces,  we  must  estimate  the  weight  of  the  rest 
also  in  ounces,  and  not  in  pounds  ;  and  in  computing  the  velo- 
cities, in  like  manner  we  should  adhere  to  the  same  staHdard 
of  meas  ire,  both  of  space  and  of  time ;  as  iov  instance,  the 
number  of  feet  in  one  second^  or  of  miles  in  one  hour. 


60  ON  THE  LAWS  OF  MOTION. 

Emily.  I  should  have  supposed,  that  the  motion  oT 
the  ball  A  was  destroyed,  because  it  had  communicated 
all  its  motion  to  B. 

Mrs.  B.  It  is  perfectly  true,  that  when  one  body 
strikes  against  another,  the  quantity  of  motion  commu- 
nicated to  the  second  body,  is  lost  by  the  first ;  but  this 
loss  proceeds  from  the  action  of  the  body  which  is  struck. 

Here  are  six  ivory  balls  hanging  in  a  row,  (fig.  4.)  draw 
the  first  out  of  the  perpendicular,  and  let  it  fall  against 
the  second.  None  of  the  balls  appear  to  move,  you  sec, 
except  the  last  which  flies  oflfas  far  as  the  first  ball  fell; 
can  you  explain  this? 

Caroline.  1  believe  so.  When  the  first  ball  struck 
the  second,  it  received  a  blow  in  retUi'n,  which  destroyed 
its  motion  ;  the  second  ball,  though  it  did  not  appear  to 
move,  must  have  struck  against  the  third  ;  the  re-actioa 
of  which  set  it  at  rest ;  the  action  of  the  third  ball  must 
have  been  destroyed  by  the  re  action  of  the  fourth,  and 
so  on  till  motion  was  communicated  to  the  last  ball, 
which,  not  being  re-acted  upon,  flies  off. 

Mrs.  B.  Very  well  explained.  Observe,  that  it  is 
only  when  bodies  are  elastic,  as  these  ivory  balls  are,  that 
the  stroke  returned  is  equal  to  the  stroke  given.  I  will 
show  you  the  difference  with  these  two  balls  of  clay,  (fig. 
5.)  which  are  not  elastic  ;  when  you  raise  one  of  these 
D,  out  of  the  perpendicular,  and  let  it  fall  against, 
the  other,  E,  the  re-action  of  the  latter,  on  account  of  its 
not  bein|i.  elastic,  is  not  suflicient  to  destroy  the  motion 
of  the  former;  only  part  of  the  motion  of  D  will  be 
con^municdted  to  E,  and  the  two  balls  wdl  move  on 
together  to  d  and  e,  \\W\c,\\  is  not  to  so  great  a  distance 
as  that  through  which  D  feU. 


ON  THE  LAWS  OF  MOTION,  61 

Observe  how  useful  re-action  is  in  nature.  Birds  in 
flying  strike  the  air  with  their  wings,  and  it  is  the  re- 
action of  the  air  which  enables  them  to  rise,  or  advance 
forwards  ;  re-action  being  always  in  a  contrary  direc- 
tion to  action. 

Caroline.  I  thought  that  birds  might  be  lighter  than 
the  air,  when  their  wings  were  expanded,  and  by  that 
means  enabled  to  fly. 

Mrs.  B.  When  their  wings  are  spread,  they  are  bet- 
ter supported  by  the  air,  as  they  cover  a  greater  extent 
of  surface ;  but  they  are  still  much  too  heavy  to  remain 
in  that  situation,  without  continually  flapping  their 
wings,  as  you  may  have  noticed,  when  birds  hover  over 
their  nests :  the  force  with  which  their  wings  strike 
against  the  air  must  equal  the  weight  of  their  bodies,  in 
order  that  i\\e  re-action  of  the  air  may  be  able  to  sup- 
port that  weight ;  the  bird  will  then  remain  stationary. 
If  the  stroke  of  the  wings  is  greater  than  is  required 
merely  to  support  the  bird,  the  re-action  of  the  air  will 
make  it  rise  ;  if  it  be  less,  it  will  gently  descend ;  and 
you  may  have  observed  the  lark,  sometimes  remaining 
with  its  wings  extended,  but  motionless  :  in  this  state 
it  drops  rapidly  into  its  nest. 

Caroline.  What  a  beautiful  effect  this  is  of  the  law 
of  re-action  !  But  if  flying  is  merely  a  mechanical  ope- 
ration, Mrs.  B.,  why  should  we  not  construct  wings, 
adapted  to  the  size  of  our  bodies,  fasten  then  to  our 
shoulders,  move  them  with  our  arms,  and  soar  into  the 
air. 

Mrs.  B.  Such  an  experiment  has  been  repeatedly  at* 
tempted,  but  never  with  success ;  and  it  is  now  con- 
sidered as  totally  impracticable.     The  muscular  power 


62  ON  THE  LAWS  OF  MOTION. 

of  birds  is  greater  in  proportion  to  their  weight  thau 
that  of  man ;  were  we  therefore  furnished  with  wings 
sufficiently  large  to  enable  us  to  flj,  we  should  not  have 
strength  to  put  them  in  motion. 

In  swimming,  a  similar  action  is  produced  on  the  wa- 
ter, as  that  on  the  air  in  flying ;  and  also  in  rowing ; 
you  strike  the  water  with  the  oars,  in  a  direction  oppo- 
site to  that  in  which  the  boat  is  required  to  move,  and  it 
is  the  re-action  of  the  water  on  the  oars  which  drives 
the  boat  along. 

Emily.  You  said,  that  it  was  in  elastic  bodies  only, 
that  re-action  was  equal  to  action;  pray  what  bodies 
are  elastic  besides  the  air? 

Mrs,  B,  In  speaking  of  the  air,  I  think  we  defined 
elasticity  to  be  a  property,  by  means  of  which  bodies 
tliat  are  compressed  returned  to  their  former  state.  If 
I  bend  this  cane,  as  soon  as  I  leave  it  at  liberty  it  re- 
covers its  former  position ;  if  I  press  my  finger  upon 
your  arm,  as  soon  as  I  remove  it,  the  flesh,  by  virtue  of 
its  elasticity,  rises  and  destroys  the  impression  I  made# 
Of  all  bodies,  the  air  is  the  most  eminent  for  this  pro- 
perty, and  it  has  thence  obtained  the  name  of  elastic 
fluid.  Hard  bodies  are  in  the  next  degree  elastic  ;  if 
two  ivory,  or  metallic  balls  are  struck  together,  the  parts 
at  which  they  touch  will  be  flattened  ;  but  their  elasti- 
city will  make  them  instantaneously  resume  their  for- 
mer shape. 

Caroline.  But  when  two  ivory  balls  strike  against 
each  other,  as  they  constantly  do  on  a  billiard  table,  no 
mark  or  impression  is  made  by  the  stroke. 

Mrs.  B,  I  beg  your  pardon ;  but  you  cannot  per- 
ceive any  mark,  because  tli^ir  elasticity  instantly  de- 
stroys all  trace  of  it. 


ON  THE  LAWS  OF  MOTION.  6S 

8oft  bodies,  which  easily  retain  impressions,  such  as 
clay,  wax,  tallow,  butter,  &c.  have  very  little  elasticity ; 
but  of  all  descriptions  of  bodies  liquids  are  the  least 
elastic. 

Emily,  If  sealins^-wax  were  elastic,  instead  of  re- 
taming  the  impression  of  a  seal,  it  would  resume  a 
smooth  surfa'^e  as  soon  as  the  weight  of  the  seal  was  re- 
moved. But  pray  what  is  it  that  produces  the  elasti- 
city of  bodies  ? 

Mrs.  B,  There  is  great  diversity  of  opinion  upon 
that  point,  and  I  cannot  pretend  to  decide  which  ap- 
proaches nearest  to  the  truth.  Elasticity  implies  sus- 
ceptibility of  compression,  and  the  susceptibility  of 
compression  depends  upon  the  porosity  of  bodies,  for 
were  there  no  pores  or  spaces  between  the  particles  of 
matter  of  which  a  body  is  composed,  it  could  not  be 
compressed. 

Caroline.  That  is  to  say,  that  if  the  particles  of 
bodies  were  as  close  together  as  possible,  they  could  not 
be  squeezed  closer. 

,Emily,  Bodies  then,  whose  particles  are  most  dis- 
tant from  each  other,  must  be  most  susceptible  of  com- 
pression, and  consequently  most  elastic ;  and  this  you 
say  is  the  case  with  air,  which  is  perhaps  the  least  dense 
of  all  bodies  ? 

Mrs,  K  You  will  not  in  general  find  this  rule  hold 
good,  for  liquids  have  scarcely  any^elasticity,  whilst  hard 
bodies  are  eminent  for  this  property,  though  the  latter 
are  certainly  of  much  greater  density  than  the  former; 
elasticity  implies,  therefore,  not  only  a  suceptibility  of 
compression,  but  depends  upon  the  power  of  resuming 
its  former  state  after  compression. 

Caroline-    But  surely  there  can  be  no  pores  in  ivory 


64>  ON  TttE  LAWS  OF  MOTION. 

and  metals,  Mrs.  B.;  how  then  can  they  be  susceptible  of 
compression  ? 

Mrs,  B,  The  pores  of  such  bodies  are  invisible  to 
the  naked  eye,  but  you  must  not  thence  conclude  that 
they  have  none  ;  it  is,  on  thl?  contrary,  well  ascertained 
that  gold,  one  of  the  most  dense  of  all  bodies,  is  ex- 
tremely porous,  and  that  tl^ese  pores  are  sufficiently 
large  to  admit  water  when  strongly  compressed  to  pass 
through  them.  This  was  showh  by  a  celebrated  expe- 
riment made  many  years  ago  at  Florence. 

Emily,  If  water  can  pass  through  gold,  there  must 
certainly  be  pores  or  interstices  which  aiford  it  a  pas- 
sage; and  if  gold  is  so  porous,  what  must  other  bodies 
be,  which  are  so  much  less  dense  than  gold  ! 

Mrs,  B,  The  chief  difference  in  this  respect  is,  I 
believe,  that  the  pores  in  some  bodies  are  Urger  than  in 
others;  in  cork,  sponge,  and  bread,  they  form  consider- 
able cavities;  in  wood  and  stone,  when  not  polished, 
they  are  generally  perceptible  to  the  naked  eye  ;  whilst 
in  ivory,  metals,  and  all  varnished  and  polished  bodies, 
they  cannot  be  discerned.  To  give  you  an  idea  of  the 
extreme  porosity  of  bodies,  sir  Isaac  Newton  conjectur- 
ed that  if  the  earth  were  so  compressed  as  to  be  abso- 
lutely without  pores,  its  dimensions  might  possibly  not 
be  more  than  a  cubic  inch. 

Caroline,  What  an  idea !  Were  we  not  indebted 
to  sir  Isaac  Newton  for  the  theory  of  attraction,  I  should 
be  tempted  to  laugh  at  him  for  such  a  supposition. 
What  insignificant  little  creatures  we  should  be  ! 

Mrs,  B,  If  our  consequence  arose  from  the  size  of 
our  bodies  we  should  indeed  be  but  pigmies,  but  remem- 
ber that  the  mind  of  Newton  was  not  circumscribed  by 
the  dimensions  of  its  envelope. 


ON  TH^  LAWS  OP  MOTION/  m 

Emily.  It  is,  however,  fortunate  that  heat  keeps  the 
pores  of  matter  open  and  distended,  and  prevents  the 
attraction  of  cohesion  from  squeezing  us  into  a  nut- 
shell. 

Mrs,  B,  Let  us  now  return  to  the  subject  of  re- 
action, on  which  we  have  some  further  observations  to 
make.  It  is  re-action,  being  contrary  to  action,  which 
produces  reflected  motwn.  If  you  throw  a  ball  against 
the  wall,  it  rebounds ;  this  return  of  the  bal  is  owing  to 
the  re-action  of  the  wall  against  which  it  struck,  and  is 
called  reflected  motion. 

Emily,  And  I  now  understand  why  balls  filled  with 
air  rebound  better  than  those  stuffed  with  bran  and  wool» 
air  being  most  susceptible  of  compression  and  most  elas- 
tic, the  re-action  is  more  complete. 

Caroline.  I  have  observed  that  when  I  throw  a  ball 
straight  against  the  wall,  it  returns  straight  to  my  hand  ; 
but  if  I  throw  it  obliquely  upwards,  it  rebounds  still 
higher,  and  I  catch  it  when  it  falls. 

Mrs,  B,  You  should  not  say  straight,  but  perpendi- 
cularly against  the  wall ;  for  straight  is  a  general  term 
for  lines  in  all  directions  which  are  neither  curved  nor 
bent,  and  is  therefore  equally  applicable  to  oblique  or 
perpendicular  lines 

Caroline.  I  thought  that  perpendicularly  meant 
either  directly  upwards  or  downwards  ? 

Mrs,  B.  In  those  directions  lines  are  perpendicular 
to  the  earth.  A  perpendicular  line  has  always  a  re- 
ference to  something  towards  which  it  is  perpendicular  ; 
tha.  is  to  say,  that  it  inclines  noither  to  the  one  side  nor 
the  other,  but  makes  an  equal  angle  on  every  side.  Do 
you  understand  what  an  angle  is  f 
3  9 


66  ON  THE  LAWS  OF  MOTION. 

Caroline,  Yes,  1  believe  so :  it  is  two  lines  meet- 
ing in  a  point. 

Mrs,  B,  Well  then,  let  the  line  A  B  (plate  II,  fig.  1.) 
represent  the  floor  of  the  room,  and  the  line  C  D 
that  in  which  jou  throw  a  ball  against  it ;  the  line  C  D 
you  will  observe,  forms  two  angles  with  the  line  A  B^ 
and  those  two  angles  are  equal. 

Emily.  How  can  the  angles  be  equal,  while  the  lines 
which  compose  them  are  of  unequal  length  ? 

Mrs,  B,  An  angle  is  not  measured  by  the  length  of 
the  lines,  but  by  their  opening. 

Emily,  Yet  the  longer  the  lines  are,  the  greater  is 
the  opening  between  them. 

Mrs,  B,  Take  a  pair  of  compasses  and  draw  a  cir- 
cle over  these  angles,  making  the  angular  point  the 
centre. 

Emily,  To  what  extent  must  I  open  the  com- 
passes. 

Mrs,  B,  You  may  draw  the  circle  what  size  you 
please,  provided  that  it  cuts  the  lines  of  the  angles  w^e 
are  to  measure.  All  circles,  of  whatever  dimensions, 
are  supposed  to  be  divided  into  360  equal  parts,  cal- 
led deji^rees  ;  the  opening  of  an  angle,  being  therefore 
a  portion  of  a  circle,  must  contain  a  certain  number 
of  degrees:  the  larger  the  angle,  the  greater  the  num- 
ber of  degrees,  and  the  two  angles  are  said  to  be  equal 
v.hen  they  contain  an  equal  number  of  degrees. 

Emily,  Now  I  understand  it.  As  the  dimensions 
of  an  angle  depend  upon  the  number  of  degrees  con- 
tained between  its  lines,  it  is  the  opening,  and  not  the 
length  of  its  lines,  which  determines  the  size  of  the 
angle* 


J^.2. 


TijJ}.  by  JXHumphrexs  ['JtOoALf^ 


ON  THE  LAWS  OF  MOTION.  §7 

Mrs.  B.  Very  well:  now  that  you  have  a  clear 
idea  of  the  dimensions  of  angles,  can  you  tell  me  how 
many  degrees  are  contained  in  the  two  angles  formed 
by  one  line  falling  perpendicular  on  another,  as  in  the 
figure  I  have  just  drawn  ? 

Emily,  You  must  allow  me  to  put  one  foot  of  the 
compasses  at  the  point  of 'the  angles,  and  draw  a  cir- 
cle round  them,  and  then  I  think  I  shall  be  able  to  an- 
swer your  question:  the  two  angles  are  together  just 
equal  to  half  a  circle,  they  contain  therefore  90  degrees 
each  ;  90  degrees  being  a  quarter  of  360. 

Mrs,  B,  An  angle  of  90  degrees  is  called  a  right  an- 
gle, and  when  one  line  is  perpendicular  to  another,  it 
forms,  you  see,  (fig.  1.)  a  right  angle  on  either  side.  An- 
gles containing  more  than  90  degrees  are  called  obtuse 
angles  (fig  2);  and  those  containing  less  than  90  de- 
grees are  called  acute  angles,  (fig.  3.) 

Caroline,  The  angles  of  this  square  table  are  right 
angles,  but  those  of  the  octagon  table  are  obtuse  angles ; 
and  the  angles  of  sharp-pointed  instruments  are  acute 
angles. 

Mrs,  B,  Very  well.  To  return  now  to  your  observa- 
tion, that  if  a  ball  is  thrown  obliquely  against  the  wall 
lit  will  not  rebound  in  the  same  direction  ;  tell  me,  have 
you  ever  played  at  billiards  ? 

Caroline,  Yes,  frequently ;  and  1  have  observed  that 
when  I  push  the  ball  perpendicularly  against  the  cushion 
it  returns  in  the  same  direction  ;  but  when  I  send  it  ob- 
liquely to  the  cushion,  it  rebounds  obliquely,  but  on  the 
opposite  side  ;  the  ball  in  this  latter  case  detscribes  an 
angle,  the  point  of  which  is  at  the  cushion.  I  have  ob- 
served too,  that  the  more  obliquely  the  ball  is  struct 


6&  ON  THE  LAWS  OP  MOTION. 

against  the  cushion,  the  more  obliquely  it  rebounds  on, 
the  opposite  side,  so  that  a  billiard  player  can  cal- 
culate with  great  accuracy  in  what  direction  it  will  re- 
turn. 

Mrs.  B.  Very  well.  This  figure  (fig.  4.  plate  II.} 
represents  a  billiard  table ;  now  if  you  draw  a  line 
A  B  from  the  point  where  the  ball  A  strikes  perpendi* 
cular  to  the  cushion ;  you  will  find  that  it  will  divide 
the  angle  which  the  ball  describes  into  two  parts,  or  iw9 
angles ;  the  one  will  show  the  obliquity  of  the  direc- 
tion of  the  ball  in  its  passage  towards  the  cushion,  the 
other  its  obliquity  in  its  passage  back  from  the  cushion. 
The  first  is  called  the  angle  of  incidence,  the  other 
the  angle  of  reflection,  and  these  angles  are  always 
equal. 

Caroline.  This  then  is  the  reason  why,  when  I  throw 
a  ball  obliquely  against  the  wall,  it  rebounds  in  an  op- 
posite oblique  direction,  forming  equal  angles  of  inci- 
dence and  of  reflection. 

Mrs.  B.  Certainly;  and  you  will  find  that  the  more 
obliquely  you  throw  the  ball,  the  more  obliquely  it 
will  rebound. 

We  must  now  conclude  ;  but  I  shall  have  some  fur- 
ther observations  to  make  upon  the  laws  of  motion,  at 
our  next  meeting. 


CONVERSATION  IV. 


ON  COMPOUND  MOTION. 

COMPOUND    MOTION,     THE    HESULT    OP    TWO    OPPOSITE    FOHCES.— ^ 
^F    CIRCPLAE    MOTION,     THE    RESULT     OP     TWO    FORCES    ONE    OF 

WHICH    CONFINES    THE    BODY    TO     A    FIXED    POINT. CENTRE    0» 

MOTION,    THE    POI»TT    AT    REST  WHILE  THE  OTHER  PARTS  OF  THE 

BODY    MOVE    ROUND    IT. CENTUE    OF    MAGNITUDE,    THE   MIDDLE 

OF   A    BODY. CENTRIPETAL   FORCE,    THAT    WHICH    CONFINES   A 

BODY    TO    A     FIXED     CENTRAL     POINT. CENTRIFUGAL     FORCE, 

THAT    WHICH    IMPELS    A    BODY    TO    FLY    FROM     THE     CENTRE. 

FALL   OF    BODIES    IN    A    PARABOLA. CENTRE    OF    GRAVITY,    THE 

CENTRE    OF    WEIGHT,    OR   POINT   ABOUT    WHICH   THE   PARTS    BA- 
JjANCE   each   OTHER, 


Mrs.  B. 

1  MUST  now  explain  to  you  the  nature  of  compound 
motion.  Let  us  suppose  a  body  to  be  struck  by  two 
equal  forces  in  opposite  directions,  how  will  it  move  ? 

Emily,  If  the  directions  of  the  forces  are  in  exact 
opposition  to  each  other,  I  suppose  the  body  would  not 
move  at  all. 

Mrs.  B,  You  are  perfectly  right;  but  if  the  forces, 
instead  of  acting  on  the  body  in  opposition,  strike  it  in 


r 


fO  ON  COMPOUND  MOTION. 

two  directions  inclined  to  each  other,  at  an  angle  of' 
ninety  degrees,  if  the  ball  A  (%.  5,  plate  II.)  be  i*truck 
bj  equal  forces  at  X  and  at  Y,  will  it  not  move  ? 

Emily.  The  force  X  would  send  it  towards  B,  and 
the  force  Y  towards  C;  and  since  these  forces  are 
equal,  I  do  hot  know  how  the  body  can  obey  one  im- 
pulse rather  than  the  other,  and  yet  I  think  the  ball 
would  move,  because  as  the  two  forces  do  not  act  in 
direct  opposition,  they  cannot  entirely  destroy  the  ef- 
fect of  each  other. 

Mrs  B.  Very  true ;  the  ball  will  therefore  follow 
the  direction  of  neither  of  the  forces,  but  will  move  in 
a  line  between  them,  and  will  reach  D  in  the  same 
space  of  time,  that  the  force  X  would  have  sent  it  to 

B,  and  the  force  Y  would  have  sent  it  to  C.  Now  if 
you  draw  two  lines  from  D,  to  join  B  and  C,  you  will 
form  a  square,  and  the  oblique  line  which  the  body  de- 
scribes is  called  the  diagonal  of  the  square. 

Caroline.  That  is  very  clear,  but  supposing  the  two 
forces  to  be  unequal,  that  the  force  X,  for  instance,  be 
twice  as  great  as  the  force  Y  ? 

Mrs.  B.  Then  the  force  X  would  drive  the  ball 
twice  as  far  as  the  force  Y,  consequently  yon  must 
draw  the  line  A  B  (fig.  6.),  twice  as  long  as  the  line  A 

C,  the  body  will  in  this  case  move  to  D ;  and  if  you 
draw  lines  from  that  point  to  B  and  C,  you  will  find 
that  the  ball  has  moved  in  the  diagonal  of  a  rectangle. 

Emily.  Allow  me  to  put  another  case  ?  Suppose  the 
two  forces  are  unequal,  but  do  not  act  on  the  ball  in 
the  direction  of  a  right  angle,  but  in  that  of  an  acute 
angle,  what  will  result  ? 


&S  COMPOUND  MOTION.  ^i. 

Mrs.  B.  Prolong  the  lines  in  the  directions  of  the 
two  forces,  and  yo«  will  soon  discover  which  way  the 
ball  will  be  impelled  ;  it  will  move  from  from  A  to  D, 
in  the  diagonal  of  a  parallelogram,  (fig.  7.)  Forces  act- 
ing in  the  direction  of  lines  forming  an  obtuse  angle, 
will  also  produce  motion  in  the  diagonal  6f  a  parallel- 
egram.  For  instance,  if  the  body  set  out  from  B,  in- 
stead of  A,  and  was  impelled  by  the  forces  X  and  V, 
it  would  move  in  the  dotted  diagonal  B  C. 

We  may  now  proceed  to  circular  motion  :  this  is 
the  result  of  two  forces  on  a  body,  by  one  of  which  it  is 
projected  forward  in  a  right  line,  whilst  by  the  other  it 
is  confined  to  a  fixed  point.  For  instance  when  1  whirl 
this  ball,  which  is  fastened  to  my  hand  with  a  string 
the  ball  moves  in  a  circular  direction  ;  because  it  is 
acted  on  by  two  forces,  that  which  I  give  it  which  re- 
presents the  force  of  projection,  and  that  of  the  string 
which  confines  it  to  my  hand.  If  during  its  motion  you 
were  suddenly  to  cut  the  string,  the  ball  would  fly  off 
in  a  straight  line;  being  released  from  confinement  to 
the  fixed  point,  it  would  be  acted  on  but  by  one  force^ 
and  motion  produced  by  one  force,  you  know,  is  al- 
ways in  a  right  line. 

Caroline.  This  is  a  little  more  difficult  to  compre- 
hend than  compound  motion  in  straight  lines. 

Mrs.  B.  You  have  seen  a  mop  tiundled,  and  have 
observed,  that  the  threads  which  compose  the  head 
of  the  mop  fly  from  the  centre ;  but  being  confined  to 
it  at  one  end,  they  cannot  part  froLa  it ;  whilst  the  wa- 
ter they  co'itain,  being  unconfined,  is  thrown  otf  in 
straight  line^. 


^  ON  COMPOUND  MOTION. 

Emily.  In  the  same  way,  the  flyers  of  a  windmili, 
When  put  in  motion  by  the  wind,  would  be  driven 
straight  forwards  in  a  right  line,  were  they  not  con- 
fined to  a  fixed  point  round  which  they  are  compelled 
to  move. 

Mrs.  B.  Very  well.  And  observe,  that  the  point 
to  which  the  motion  of  a  small  body,  such  as  the  ball 
with  the  string,  which  may  be  considered  as  revolving 
in  one  plane,  is  confined,  becomes  the  centre  of  its  mo- 
tion. But  when  the  bodies  are  not  of  a  size  or  shape 
to  allow  of  our  considering  every  part  of  them  as  mov- 
ing in  the  same  plane,  they  in  reality  revolve  round  a 
line,  which  line  is  called  the  aocis  of  motion.  In  a  top, 
for  instance,  when  spinning  on  its  point,  the  axis  is  the 
line  which  passes  through  the  middle  of  it,  perpendicu- 
larly to  the  floor. 

Caroline.  The  axle  of  the  flyers  of  the  windmill,  is 
then  the  axis  of  its  motion  ;  but  is  the  centre  of  motion 
always  in  the  middle  of  a  body  ? 

Mrs.  B.  No,  not  always.  The  middle  point  of  a 
body,  is  called  its  centre  of  magnitude,  or  position,  that 
is  the  centre  of  its  mass  or  bulk.  Bodies  have  also 
another  centre,  called  the  centre  of  gravity,  which  I 
shall  explain  to  you  ;  but  at  present  we  must  confine 
ourselves  to  the  axis  of  motion.  This  line  you  must  ob- 
serve remains  at  rest,  whilst  all  the  other  parts  of  the 
body  move  around  it;  when  you  spin  a  top  the  axis  is 
stationary  whilst  every  other  part  is  in  motion  round  it. 

Caroline.  But  a  top  generally  has  a  motion  forwards, 
besides  its  spinning  motion ;  and  then  no  point  within 
ft  can  be  at  rest  ? 


0 


Pul).  by  J.Y.HiunjiluY.ys  rhiLi.l-^ 


UN  COMPOUND  MOTION.  7o 

Mrs.  B.  What  I  say  of  the  axis  of  motion,  relates 
@nlj  to  circular  motion ;  that  is  to  say,  to  motion  round 
a  line,  and  not  to  that  which  a  body  may  have  at  the 
same  time  in  any  other  direction.  There  is  one  cir- 
cumstance in  circular  motion,  which  you  must  careful- 
ly attend  to;  which  is,  that  the  further  any  part  of  a 
body  is  from  the  axis  of  motion,  the  greater  is  its  velo- 
city ;  as  you  approach  that  line,  the  velocity  of  the  parts 
gradually  diminish  till  you  reach  the  axis  of  motion, 
which  is  perfectly  at  rest. 

Caroline  .But,  if  every  part  of  the  same  body  did  not 
move  with  the  same  velocity,  that  part  which  moved 
quickest,  must  be  separated  from  the  rest  of  the  body, 
and  leave  it  behind  ? 

Mrs,  B.  You  perplex  yourself  by  confounding  the 
idea  of  circular  motion,  with  that  of  motion  in  a  right 
line;  you  must  think  only  of  the  motion  of  a  body 
round  a  fixed  line,  and  you  will  find,  that  if  the  parts 
farthest  from  the  centre  had  not  the  greatest  velocity 
those  parts  would  not  be  able  to  keep  up  with  the  rest 
of  the  body,  and  would  be  left  behind.  Do  not  the  ex- 
tremities of  the  vanes  of  a  windmill  move  over  a  much 
greater  space,  than  the  parts  nearest  the  axis  of  mo- 
tion ?  (pi. III.  fig.  1.)  the  three  dotted  circles  describe  the 
paths  in  which  three  difterent  parts  of  the  vanes  move, 
and  though  the  circles  are  of  different  dimensions  the 
vanes  describe  each  of  them  in  the  same  space  of  time. 

Caroline,  Certainly  they  do;  and  1  now  only  won- 
der, that  we  neither  of  us  ever  made  the  observation 
before :  and  the  same  effect  must  take  place  in  a  solid 
body,  like  the  top  in  spinning;  the  most  bulging  part  of 
the  surface  must  move  with  the  greatest  rapidity. 

G 


74  ON  COMPOUND  MOTION. 

Mrs,  B.  The  force  which  confines  a  body  to  a  cen- 
tre, round  which  it  moves  is  called  the  centripetal  force; 
and  that  force,  which  impels  a  body  to  Hy  from  the  cen-? 
tre,  is  called  *he  centrifugal  force  ;  in  circular  motion 
these  two  forces  constantly  balance  each  other  ;  other- 
wise the  revolving  body  would  either  approach  the 
centre  or  recede  from  it,  according  as  the  one  or  the 
ether  prevailed. 

Caroline,  When  I  see  any  body  moving  in  a  circle, 
I  shall  remember,  that  it  is  acted  on  by  two  forces. 

'Mrs,  B.  Motion,  either  in  a  circle,  an  ellipsis,  or  any 
other  curve-line,  must  be  the  result  of  the  action  of  two 
forces  ;  for  you  know,  that  the  impulse  of  one  single 
force,  always  produces  motion  in  a  right  line. 

Emily,  And  if  any  cause  should  destroy  the  centri- 
petal force,  the  centrifugal  force  would  alone  impel  the 
body,  and  it  would  I  suppose  fly  otT  in  a  straight  line 
from  the  centre  to  which  it  had  been  confined. 

Mrs,  B,  It  would  not  fly  off  in  a  right  line  from  the 
centre ;  but  in  a  right  line  in  the  direction  in  which  it 
was  moving,  at  the  instant  of  its  release  ;  it  a  stone, 
whirled  round  in  a  sling,  gets  loose  at  the  point  A» 
(plate  III.  fig.  2.)  it  flies  off  in  the  direction  A  B  ;  this 
line  is  called  a  tangent,  it  touches  the  circumference  of 
the  circle,  and  forms  a  right  angle  with  a  line  drawn 
from  that  point  of  the  circumference  to  the  centre  of 
the  circle,  C. 

Emily,  You  say,  that  motion  in  a  curve-line,  is 
owing  to  two  forces  acting  upon  a  body;  but  when  I  throw 
this  ball  in  an  horizontal  direction,  it  describes  a  curve 
line  in  falling ;  and  yet  it  is  only  acted  upon  by  the  force 
of  projection  ;  there  is  no  centripetal  force  to  confine  it, 
or  produce  compound  motion. 


ON  COMPOUND  MOTION.  7S 

Mrs*  B,  A  ball  thus  tlv.own,  is  acted  upon  by  no 
less  tl.an  three  forces ;  the  force  of  projection,  which 
you  communicated  to  it  ;  the  resistance  of  the  air 
through  which  it  passes,  which  diminishes  its  velocity, 
without  chanj^in^  its  direction  ;  and  the  force  of  gravity, 
'which  finally  brings  it  to  the  ground.  The  power  of 
gravity,  a'id  the  resistance  of  the  air,  being  always 
greater  than  any  force  of  projection  we  can  give  a  body, 
the  latter  is  gradually  overcome,  and  the  body  brought 
to  the  ground  ;  but  the  stronger  the  projectile  force,  the 
longer  will  these  powers  be  in  subduing  it,  and  the  fur- 
ther the  body  will  go  before  it  falls. 

Caroline.  A  shot  fired  from  a  cannon,  for  instance, 
wil4  go  much  further,  than  a  stone  projected  by  the 
iiand. 

Mrs.  B,  Bodies  thus  projected,  you  observed,  des- 
cribed a  curve-line  in  their  descent ;  can  you  account 
for  that  ? 

Caroline,  No  ;  I  do  not  understand,  why  it  should 
not  fall  in  the  diagonal  of  a  square. 

Mrs.  B.  You  must  (consider  that  the  force  of  projec- 
tion is  strongest  when  the  ball  is  first  thrown ;  this 
force,  as  it  proceeds,  being  weakened  by  the  continued 
resistance  of  the  air,  the  stone,  therefore,  begins  by 
moving  in  an  horizontal  direction ;  but  as  the  stronger 
powers  prevail,  the  direction  of  the  ball  will  gradually 
change  from  an  horizontal,  to  a  perpendicular  line. 
Projection  alone,  would  drive  the  ball  A,  to  B,  (fig.  3.) 
gravity  would  bring  it  to  C ;  therefore,  when  acted  on 
in  different  directions,  by  these  two  forces,  it  moves  be« 
tween,  gradually  inclining  more  and  more  to  the  force 
of  gravity,  in  proportion  as  this  accumulates  ;  instead 


76  ON  COMPOUND  MOTION. 

therefore  of  reaching  the  ground  at  D,  as  jou  suppose 
it  would,  it  falls  somewhere  about  E. 

Caroline,  It  is  precisely  so ;  look,  Emily,  as  I  throw 
this  ball  directly  upwards,  how  the  resistance  of  the  air 
and  gravity  conquers  projection.  Now  I  will  throw  it 
upwards  ubliquely  :  see  the  force  of  projection  enables  ' 
it,  for  an  instant,  to  act  in  opposition  to  that  of  gravity  ; 
but  it  is  soon  brought  down  again. 

Mrs,  B,  The  curve-line  which  the  ball  has  describ- 
ed, is  called  in  geometry  -d.  parabola  ^  but  when  the  ball 
is  thrown  perpendicularly  upwards,  it  will  descend  per- 
pendicularly ;  because  the  force  of  projection,  and  that 
of  gravity,  are  in  the  same  line  of  direction. 

We  have  noticed  the  centres  of  magnitude,  and-  of 
motion  ;  but  I  have  not  yet  explained  to  you,  what  is 
meant  by  the  centre  of  gravity;  it  is  that  point  in  a 
body,  about  which  all  the  parts  exactly  balance  each 
other;  if  therefore  that  point  is  supported,  the  body 
will  not  fall.     Do  you  understand  this  ? 

Emily,  I  think  so,  if  the  parts  round  about  this 
point  have  an  equal  tendency  ^to  fall,  they  will  be  in 
equilibrium,  and  as  long  as  this  point  is  supported,  the 
body  cannot  fall. 

Mrs.  B,  Caroline,  what  would  be  the  effect,  were 
any  other  point  of  the  body  alone  supported  ? 

Caroline,  The  surrounding  parts  no  longer  balanc- 
ing each  other,  the  body,  I  suppose,  would  fall  on  the 
side  at  which  the  parts  are  heaviest. 

Mrs,  B,  Infallibly  ;  whenever  the  centre  of  gravity 
is  unsupported,  the  body  must  fall.  This  sometimes 
happens  with  an  overloaded  waggon  winding  up  a  steep 
hill,  one  side  of  the  road  being  more  elevated  than  th« 


ON  COMPOUND  MOTION.  77 

©ther;  let  us  suppose  it  to  slope  as  is  described  in  this 
figure,  (plate  III.  fig.  4.,)  we  will  sav,  that  the  centre  of 
gravity  of  this  loaded  wa2;gon  is  at  the  point  A.  Now 
your  eye  will  tell  you,  that  a  waggon  thus  situated,  will 
overset;  and  the  reason  is,  that  the  centre  of  gravity 
A,  is  not  supported ;  for  if  you  draw  a  perpendicular 
line  from  it  to  the  ground  at  C,  it  does  not  fall  under 
the  waggon  within  the  wheels,  and  is  therefore  not  sup- 
ported by  tliem. 

Caroline.  I  understand  ihat  perfectly ;  but  what  is 
the  meaning  of  the  other  point  B  ? 

Mrs,  B,  Let  us,  in  imagination  take  off  the  upper 
part  of  the  load  ;  the  centre  of  gravity  will  then  change 
its  situation,  and  descend  to  B,  as  that  will  now  be  the 
point  about  ^hich  the  parts  of  the  less  heavily  laden 
waggon  will  balance  each  other.  Will  the  waggon  nov/ 
be  upset? 

Caroline.  No,  because  a  perpendicular  line  from 
that  point  falls  within  the  wheels  at  D,  and  is  supported 
by  them  ;  and  when  the  centre  of  gravity  is  supported, 
the  body  will  not  fall. 

Emily.  Yet  I  should  not  much  like  to  pass  a  wag- 
gon, in  that  situation  ;  for,  as  you  see,  the  point  D  is  but 
just  within  the  left  wheel ;  if  the  right  wheel  was  merely 
raised,  by  passing  over  a  stone,  the  point  D  would  be 
thrown  on  the  outside  of  the  left  wheel,  and  the  waggon 
would  upset. 

Caroline.     A  waggon,  or  any  carriage  whatever,  will 
then  be  most  firmly  supported,  Vvhen  the  centre  of  gra- 
vity falls  exactly  between  the  wheels  ;  and  that  is  the 
case  in  a  level  road. 
G   2 


rS  ON  COMPOUND  MOTION. 

Pray,  whereabouts  is  the  centre  of  gravity  of  the 
human  body  ? 

Mrs,  B.  between  the  hips ;  and  as  long  as  we  stand, 
upright,  this  point  is  supported  by  the  feet ;  if  you  lean 
on  one  side,  you  will  find  that  you  no  longer  stand 
llrm.  A  rope-dancer  performs  all  his  feats  of  agility, 
by  dexterously  supporting  his  centre  of  gravity  ;  when- 
ever he  finds  that  he  is  in  danger  of  losing  his  ba- 
lance, he  shifts  the  heavy  pole,  which  he  holds  in 
his  hands,  in  order  to  throw  the  weight  towards  the 
side  that  is  deficient;  and  thus  by  changing  the  situa- 
tion of  the  centre  of  gravity,  he  restores  his  equilibrium. 

Caroline,  When  a  stick  is  poised  on  the  tip  of  the 
finger,  is  it  not  by  supporting  its  centre  of  gravity  ? 

Mrs.  B.  Yes  ;  and  it  is  because  the  centre  of  gra- 
vity is  not  supported,  that  spherical  bodies  roll  down 
a  slope.  A  sphere  being  perfectly  round,  can  touch  the 
slope  but  by  a  single  point,  and  that  point  cannot  be 
perpendicularly  under  the  centre  of  gravity,  and  there- 
fore cannot  be  supported,  as  you  will  perceive  by  ex- 
amining this  figure.  (Jig,  5.  plate  III.) 

Emily,  So  it  appears ;  yet  I  have  seen  a  cylinder 
of  wood  roll  up  a  slope  ;  how  is  that  contrived  } 

Mrs,  B,  It  is  done  by  plugging  one  side  of  the  cyl- 
inder with  lead,  as  at  B.  (fig.  5.  plate  III.)  the  body  be- 
ing no  longer  of  an  uniform  density,  the  centre  of  gra- 
vity is  removed  from  the  middle  of  the  body  to  some 
point  in  the  lead,  as  that  substance  is  much  heavier 
than  wood ;  now  you  may  observe  that  in  order  that 
the  cylinder  may  roll  down  the  plane,  as  it  is  here  si- 
tuated, the  centre  of  gravity  must  rise,  which  is  impos- 
sible; the  centre  of  gravity    must  always  descend  in 


ON  COMPOUND  MOTION.  79 

moving,  and  will  descend  bj  the  nearest  and  readiest 
means,  which  will  be  by  forcing  the  cylinder  up  the 
slope,  until  the  centre  of  gravity  is  supported,  and 
then  it  stops. 

Caroline,  The  centre  of  gravity,  therefore,  is  not 
always  in  the  middle  of  a  body. 

Mrs,  B,  No,  that  point  we  have  called  the  centre 
of  magnitude  ;  when  the  body  is  of  an  uniform  density 
the  centre  of  gravity  is  in  the  same  point  ;  but  when 
one  part  of  the  body  is  composed  of  heavier  materials 
than  another  part,  the  centre  of  gravity  being  the  centre 
of  the  weight  of  the  body  can  no  longer  correspond  with 
the  centre  of  magnitude.  Thus  you  see  the  centre  of 
gravity  of  this  cylinder  plugged  with  lead,  cannot  be 
in  the  same  spot  as  the  centre  of  magnitude. 

Emily,  Bodies,  therefore,  consisting  but  of  one 
kind  of  substance,  as  wood,  stone,  or  lead,  and  whose 
densities  are  consequently  uniform,  must  stand  more 
firmly,  and  be  more  difficult  to  overset,  than  bodies 
composed  of  a  variety  of  substances,  of  different  densi- 
ties, which  may  throw  the  centre  of  gravity  on  one 
side. 

Mrs,  B,  Yes  ;  but  there  is  another  circumstance 
which  more  materially  affects  the  firmness  of  their  posi- 
tion, and  that  is  their  form.  Bodies  that  have  a  narrow 
base  are  easily  upset,  for  if  they  are  the  least  inclined, 
their  centre  is  no  longer  supported,  as  you  may  per- 
ceive in  fig.  6. 

Caroline,  I  have  often  observed  with  what  difficul- 
1y  a  person  carries  a  single  pail  of  water;  it  is  owing, 
I  suppose,  to  the  centre  of  gravity  being  thrown  on  one 
side,  and  the  opposite  arm  is  stretched  out  to  endeaveur 


so  ON  COMPOUND  MOTION* 

to  bring  it  back  to  its  original  situation  ;  but  a  pail  hang? 
ing  on  each  arm  is  carried  without  difficulty,  because 
they  balance  each  other,  and  the  centre  of  gravity  re- 
mains supported  by  the  feet. 

Mts,  B.  Very  well ;  I  have  but  one  more  remark  to 
make  on  the  centre  of  gravity,  which  is,  that  when  two 
bodies  are  fastened  together,  by  a  line,  string,  chain, 
or  any  power  whatever,  they  are  to  be  considered  as 
forming  but  one  body ;  if  the  two  bodies  be  of  equal 
weight,  the  centre  of  gravity  will  be  in  the  middle  of  the 
line  which  unites  them,  (fig.  7.)  but  if  one  be  heavier  than 
the  other,  the  centre  of  gravity  will  be  proportion- 
ally nearer  the  heavy  body  than  the  light  one.  (fig.  8). 
If  you  were  to  carry  a  rod  or  pole  with  an  equal  weight 
fastened  at  each  end  of  it,  you  would  hold  it  in  the  mid- 
dle of  the  rod,  in  order  that  the  weights  should  ba- 
lance each  other ;  whilst  if  it  had  unequal  weights  at 
each  end  you  would  hold  it  nearest  the  greater  weight, 
to  make  them  balance  each  other. 

Emily,  And  in  both  cases  we  should  support  the 
centre  of  gravity  ;  and  if  one  weight  be  very  consi- 
derably larger  than  the  other,  the  centre  of  gravity  will 
be  thrown  out  of  the  rod  into  the  heaviest  weight, 
(fig.  9.) 

Mrs,  B.    Undoubtedly. 


CONVERSATION  V. 


ON  THE  MECHANICAL  POWERS. 


©r    THE    POWER    OF    MACHINES. OF    THE    LEVER  IN    GENERAL — OF 

THE  LEVER  OF  THE  FIRST  KIND,  HAVING  THE  FULCRUM  BE- 
TWEEN THE  POWER  AND  THE  M^EIGHT.— OF  THE  LEVER  OF 
THE  SECOND  KIND,  HAVING  THE  WEIGHT  BETWEEN  THE  POW- 
ER   AND    THE     FULCRUM. OP    THE    LEVER    OF    THE    THIRD    KIND, 

HAVING  THE  POWER  BETWEEN  THE  FULCRUM  AN5  THE 
WEIGHT. 


Mrs.  B. 

We  may  now  proceed  to  examine  the  mechanical- 
powers;  they  are  six  in  number,  one  or  more  of  which 
enters  into  the  composition  of  every  machine.  The 
lever,  the  pulley,  the  wheel  and  axle,  the  inclined 
plane,  the  wedge,  and  the  screw. 

In  order  to  understand  the  power  of  a  machine,  there 
are  four  things  to  be  considered.  1st.  The  power  that 
acts :  this  consists  in  the  eifort  of  men  or  horses,  of 
weights,  springs,  steam,  &c. 

2dly.  The  resistance  which  is  to  be  overcome  by  the 
power ;  this  is  generally  a  weight  to  be  meved.     The 


^2  ON  THE  MECHANICAL  POWERS. 

power  must  always  be  superior  to  the  resistance,  other- 
wise the  machine  could  not  be  put  in  motion. 

Caroline.  If  for  instance  the  resistance  of  a  carri- 
age was  greater  than  the  strength  of  the  horses  emjjloy- 
ed  to  draw  it,  they  would  not  be  able  to  make  it  m^)ve. 

Mrs.  B.  3dly.  We  are  to  consider  the  centre  of 
motion,  or  as  it  is  termed  in  mechanics,  the  fulcrum  ; 
this  you  may  recollect  is  the  point  about  which  all  the 
parts  of  the  body  move  ;  and  lastly,  the  respective  ve- 
locities of  the  power,  and  of  the  resistance. 

Emily.  That  must  depend  upon  their  respective 
distances  from  the  axis  of  motion  ;  as  we  observed  in 
the  motion  of  the  vanes  of  the  windmill. 

Mrs,  B.  We  shall  now  examine  the  power  of  the 
Je*^^r.  The  lever  is  an  inflexible  rod  or  beam  of  any 
kind,  that  is  to  say,  one  which  will  not  bend  in  any 
direction.  Wr  instance,  the  steel  rod  to  which  these 
scales  are  suspended  is  a  lever,  and  the  point  in  which 
it  is  supported  the  fulcrum,  or  centre  of  motion  ;  now, 
can  you  tell  me  why  the  two  scales  are  in  equilibrium  ? 

Caroline.  Being  both  empty,  and  of  the  same  weighty 
they  balance  each  other. 

Emily.  Or,  more  correctly  speaking,  because  the 
centre  of  gravity  common  to  both  is  supported. 

Mrs.  B,  Very  well ;  and  which  is  the  centre  of  gra- 
vity of  this  pair  of  scales  ?  (fig.  1.  plate  IV,) 

Emily.  You  have  told  us  that  when  two  bodies  of 
equal  weight  were  fastened  together,  the  centre  of  gra- 
vity was  in  the  middle  of  the  line  that  connected  them; 
the  centre  of  gravity  of  the  scales  must  therefore  be  in 
the  fulcrum  F  of  the  lever  which  unites  the  two  scales  ; 
and  corresponds  with  the  centre  of  motion. 


«/*.  7jvJ.Y.H>jjia>l,rrxs  /'A/Ay.iV 


ON  THE  MECHANICAL  POWERS.  83 

Caroline.  But  if  the  scaks  contained  different 
weights,  the  centre  of  gravity  would  no  longer  be  in  the 
fulcrum  of  the  lever,  but  removed  towards  that  scale 
which  contained  the  heaviest  weight  ;  and  since  that 
point  would  no  longer  be  supported,  the  heavy  scale 
would  descend  and  out-weigh  the  other. 

Mrs.  B.  True;  but  tell  me,  can  you  imagine  any 
mode  by  which  bodies  of  different  weights  can  be  made 
to  balance  each  other,  either  in  a  pair  of  scales,  or  sim- 
ply suspended  to  the  extremities  of  the  lever  ?  for  the 
scales  are  not  an  essential  part  of  the  machine,  they 
have  no  mechanical  power,  and  are  used  merely  for  the 
convenience  of  containing  the  substance  to  be  weighed. 
Caroline.  What !  make  a  light  body  balance  a  hea- 
vy one  ?  I  cannot  conceive  that  possible. 

Mrs.  B.  The  fulcrum  of  this  pair  of  scales  (fig.  2.) 
is  moveable,  you  see ;  I  can  take  it  off  the  prop,  and  fas- 
ten it  on  again  in  another  part ;  this  part  is  now  become 
the  fulcrum,  but  it  is  no  longer  in  the  centre  of  the 
lever. 

Caroline^  And  the  scales  are  no  longer  true;  for 
that  which  hangs  on  the  longest  side  of  the  lever^de- 
scends. 

Mrs,  B,  The  two  parts  of  the  lever  divided  by  the 
fulcrum  are  called  its  arms,  you  should  therefore  say 
the  longest  arm,  not  the  longest  side  of  the  lever.  These 
arms  are  likewise  frequently  distinguished  by  the  ap- 
pellations of  the  acting  and  the  resisting  part  of  the 
lever. 

Your  observation  is  true  that  the  balance  is  now  de- 
stroyed ;  but  it  will  answer  the  purpose  of  enabling  you 


S4  ON  THE  MECHANICAL  POWERS. 

to  comprehend  the  power  of  a  lever  when  the  fulcrum 
is  not  in  the  centre. 

Emily.  This  would  be  an  excellent  contrivance  for 
those  who  cheat  in  the  weight  of  their  goods;  by  mak- 
ing the  fulcrum  a  little  on  one  side,  and  placing  the 
goods  in  the  scale  which  is  suspended  to  the  lonj^est 
arm  of  the  lever,  they  would  appear  to  weigh  more  than 
they  do  in  reality. 

Mrs,  B.  You  do  not  consider  how  easily  the  fraud 
would  be  detected  ;  for  on  the  scales  being  emptied 
they  would  not  hang  in  equilibrium. 

Emily.  True  ;  I  did  not  think  of  that  circumstance. 
But  I  do  not  understand  why  the  longest  arm  of  the 
lever  should  not  be  in  equilibrium  with  the  other? 

Caroline.  It  is  because  it  is  heavier  than  the  short- 
est arm;  the  centre  of  gravity,  therefore,  is  no  longer 
supported. 

Mrs.  B.  You  are  right ;  the  fulcrum  is  no  longer  in 
the  centre  of  gravity ;  but  if  we  can  contrive  to  make 
the  fulcrum  in  its  present  situation  become  the  centre 
of  gravity,  the  scales  will  again  balance  each  other;  for 
you  Tecollect  that  the  centre  of  gravity  is  that  point 
about  which  every  part  of  the  body  is  in  equilibrium. 

Emily.  It  has  just  occurred  to  me  how  this  may  be 
accomplished;  put  a  great  weight  into  the  scale  sus- 
pended to  the  shortest  arm  of  the  lever,  and  a  smaller 
one  into  that  suspended  to  the  longest  arm.  ii^es,  I  have 
discovered  it — look,  Mrs.  B.,  the  scale  on  tlie  shortest 
arm  will  carry  2lbs.,  and  that  on  the  longest  arm  only 
one,  to  restore  the  balance,  (fig.  3.) 

Mrs.  B.  You  see,  therefore,  that  it  is  not  so  im- 
practicable as  you  imagined  to  make  a  heavy  body  ba- 


QN  THE  MECHANICAL  POWERS.  85 

lance  a  light  one ;  and  this  is  in  fact  the  means  bj 
which  you  thought  an  imposition  in  the  weight  of  goods 
might  be  effected,  as  a  weight  of  ten  or  twelve  ounces 
might  thus  be  made  to  balance  a  pound  of  goods.  Let 
us  now  take  off  the  scales  that  we  may  consider  the 
lever  simply  ;  and  in  this  state  you  see  that  the  ful- 
crum is  no  longer  the  centre  of  gravity ;  but  it  is,  and 
must  ever  be,  the  centre  of  motion,  as  it  is  the  only 
point  which  remains  at  rest,  while  the  other  parts  move 
about  it. 

Caroline,  It  now  resembles  the  two  opposite  vanes 
of  a  windmill,  and  the  fulcrum  the  point  round  which 
they  move. 

Mrs.  B.  In  describing  the  motion  of  those  vanes, 
you  may  recollect  our  observing  that  the  farther  a  body 
is  from  the  axis  of  motion,  the  greater  is  its  velocity. 

Caroline,  That  I  remember  and  understood  per- 
fectly. 

•Mrs,  B,  You  comprehend  then,  that  the  extremity 
of  the  longest  arm  of  a  lever  must  move  with  greater 
velocity  than  that  of  the  shortest  arm  ? 

Emily,  No  doubt,  because  it  is  farthest  from  the 
centre  of  motion.  And  pray,  Mrs.  B.,  when  my  bro- 
thers play  at  see-saw,  is  not  the  plank  on  which  they 
ride  a  kind  of  lever  ? 

Mrs,  B.  Certainly  ;  the  log  of  wood  which  supports 
it  from  the  ground  is  the  fulcrum,  and  those  who  ride 
represent  the  power  and  the  resistance  at  each  end  of 
the  lever.  And  have  you  not  observed  that  when  those 
who  ride  are  of  equal  weight,  the  plank  must  be  sup- 
ported in  the  middle  to  make  the  two  arms  equal  ; 
whilst  if  the  persons  differ  in  weight,  the  plank  must 

H 


86  t)N  THE  MECHANICAL  POWERS. 

be  drawn  a  little  further  over  the  prop,  to  make  the 
arms  unequal,  and  the  lightest  person  who  represents 
the  resistance,  must  be  placed  at  the  extremity  of  the 
longest  arm. 

Caroline.  That  is  always  the  case  when  I  ride  on  a 
plank  with  my  youngest  brother;  I  have  observed  also 
that  the  lightest  person  has  the  best  ride,  as  lie  moves 
b«>th  further  and  quicker  ;  and  I  now  understand  that  it 
is  because  he  is  more  distant  from  the  centre  of  motion. 

Mrs,  B,  The  greater  velocity  with  which  your  lit- 
tle brother  moves,  renders  his  momentum  equal  to 
yours. 

Caroline.  Yes ;  I  have  the  most  gravity,  he  the 
greatest  velocity ;  so  that  upon  the  whole  our  momen- 
tums  are  equal. — But  you  said,  Mrs.  B.,  that  the  power 
should  be  greater  than  the  resistance  to  put  the  machine 
in  motion  ;  how  then  can  the  plank  move  if  the  mo- 
mentums  of  the  persons  who  ride  are  equal. 

Mrs.  B.  Because  each  person  at  his  descent  touches 
the  ground  with  his  it^i;  the  reaction  of  which  gives 
him  an  impulse  which  increases  his  velocity ;  this 
spring  is  requisite  to  destroy  the  equilibrium  of  the 
power  and  the  resistance,  otherwise,  the  plank  would 
not  move.  Did  you  ever  observe  that  a  lever  describes 
the  arc  of  a  circle  in  its  motion  ? 

Emily.  No ;  it  appears  to  me  to  rise  and  descend 
perpendicularly  ;  at  least  I  always  thought  so. 

Mrs.  B.  I  believe  I  must  make  a  sketch  of  you  and 
your  brother  riding  on  a  plank,  in  order  to  convince  you 
of  vour  error,  (fig.  4.  pi.  IV.)  You  may  now  observe  that 
a  lever  can  move  only  round  the  fulcrum,  since  that  is 
Che  centre  of  motion  ;  it  would  be  impossible  for  you  io 


ON  THE  MECHANICAL  POWERS.  87 

lise  perpendicularly  to  the  point  A,  or  for  your  brother 
to  ilescend  in  a  straight  line  to  the  point  B  ;  you  must 
in  risinjj  and  he  in  descending  describe  arcs  of  your  re- 
spective circles.  I'his  drawing  shows  you  als«  how 
much  sup'  rior  his  velocity  mu^t  be  to  yours  ;  for  if  you 
could  swinu;  quite  roUnd,  you  would  each  complete 
your  respiective  circles  in  the  same  time. 

Caroline.  My  brother's  circle  being  much  the  largest 
he  must  undoubtedly  move  the  quickest. 

Mrs,  B,  Now  tell  m<^,  do  you  think  that  your  bro- 
ther could  raise  you  as  easily  without  the  aid  of  a 
lever  ? 

Caroline,     Oh  no,  he  could  not  lift  me  off  the  ground. 

Mrs,  B.  Then  I  think  you  require  no  further  proof 
•f  the  power  of  a  lever,  since  you  see  what  it  enables 
your  brother  to  perform. 

Caroline.  I  now  understand  what  you  meant  by  say- 
ing, that  in  mechanics,  motion  was  opposed  to  matter, 
for  it  is  my  brother's  velocity  which  overcomes  my 
weight. 

Mrs.  B.  You  may  easily  imagine,  what  enormous 
weights  may  be  raised  by  levers  of  this  description,  for 
the  longer  the  acting  part  of  the  lever  in  comparison 
to  the  resisting  part,  the  greater  is  the  effect  produced 
by  it ;  because  the  greater  is  the  velocity  of  the  power 
compared  to  that  of  the  weight. 

There  are  three  different  kinds  of  levers ;  in  the 
first  the  fulcrum  is  between  the  power  and  the  weight. 

Caroline.  This  kind  then  comprehends  the  several 
levers  you  have  described. 

Mrs,  B,  Yes,  when  in  levers  of  the  first  kind,  the 
fulcrum  is  equally  between  the  power  and  the  weiglit, 


88  ON  THE  MECHANICAL  POWERS. 

as  in  the  balance,  tlie  poMTJ'  must  be  greater  than  the 
weight,  in  order  to  move  it ;  for  nothing  can  in  this  case 
be  gained  by  velocity ;  the  two  arms  of  the  lever  being 
equal,  the  velocity  of  their  extremities  must  be  so  like- 
wise. The  balance  is  therefore  of  no  assistance  as  a  me- 
chanical power,  but  it  is  extremely  useful  to  estimate 
the  respective  weights  of  bodies.  I 

But  when  (fig.  5.)  the  fulcrum  F  of  a  lever  is  not 
equally  distant  from  the  power  and  the  weight,  and  that 
the  power  P  acts  at  the  extremity  of  the  longest  arm, 
it  may  be  less  than  the  weight  W,  its  deficiency  being 
compensated  by  its  superior  velocity ;  as  we  observed 
in  the  seesaw. 

Emily.  Then  when  we  want  to  lift  a  great  weight, 
we  must  fasten  it  to  the  shortest  arm  of  a  lever,  and 
apply  our  strength  to  the  longest  arm  ? 

Mrs,  B,  If  the  case  will  admit  of  your  putting  the 
<^nd  of  the  lever  under  the  weight,  no  fastening  will  be 
required  ;  as  you  will  perceive  by  stirring  the  fire. 

Mmlly,  Oh  yes  !  the  poker  is  a  lever  of  the  first  kind, 
the  point  where  it  rests  against  the  bars  of  the  grate 
whilst  1  am  stirring  the  fire,  is  the  fulcrum  ;  the  short 
arm  or  resisting  part  of  the  lever  is  employed  in  lifting 
the  weight,  which  is  the  coals,  and  my  hand  is  the  pow- 
er applied  to  the  longest  arm,  or  acting  part  of  the 
lever. 

.Mrs,  B.  Let  me  hear,  Caroline,  whether  you  can 
equally  well  explain  this  instrument,  which  is  compo- 
sed of  two  levers,  united  in  one  common  fulcrum. 

Caroline,     A  pair  of  scissors  ! 

Mrs,  B,  You  are  surprised,  but  if  you  examine  their 


©N  THE  MECHANICAL  J»OWERS.  89 

construction,  you  will  discover  that  it  is  the  power  of 
the  lever  that  assists  us  in  cutting  with  scissors. 

Caroline.  Yes  ;  I  now  perceive  that  the  point  at 
which  the  two  levers  are  screwed  to<^ether,  is  the  ful- 
crum ;  the  handles,  to  which  the  power  of  the  fingers  is 
applied,  are  the  extremities  of  the  acting  part  of  the 
levers,  and  the  cutting  part  of  the  scissors,  are  the  re- 
sisting parts  of  the  levers :  therefore,  the  longer  the 
handles  and  the  shorter  the  points  of  the  scissors,  the 
more  easily  you  cut  with  them. 

Emily.  That  I  have  often  observed,  for  when  I  cut 
pasteboard  or  any  hard  substance,  I  always  make  use  of 
ihat  part  of  the  scissars  nearest  the  screw  or  rivet,  and 
I  now  understand  why  it  increases  the  power  of  cutting; 
but  I  confess  that  I  never  should  have  discovered  scis- 
sors to  have  been  double  levers  ;  and  pray  are  not  snuf- 
fers levers  of  a  similar  description  ? 

J\frs,  B,  Yes,  and  most  kinds  of  pincers ;  the  great 
power  of  which  consists  in  the  resisting  part  of  the 
lever  being  very  short  in  comparison  of  the  acting  part. 

Caroline,  And  of  what  nature  are  the  two  other 
kinds  of  levers  ? 

Mrs,  B,  In  levers  of  the  second  kind,  thewe'i^htj 
iastead  of  being  at  one  end,  is  situated  between  the 
power  and  the  fulcrum,  (fig.  6.) 

Caroline,  The  weight  and  the  fulcrum  have  here 
changed  places ;  and  what  advantage  is  gained  by  this 
kind  of  lever  ? 

Mrs,  B,    In   moving  it,  the  velocity  of  the  power 

must  necessarily  be  greater  than  that  of  the  weight, 

as  it  is  more  distant  from  the   centre  of  the   motion. 

Have  you  ever   seen  your  brother   move  a  snow-bail 

H  2 


90  ON  THE  MECHANICAL  POWERS. 

by  means  of  a  strong  stick,  when  it  became  too  heavy 
for  him  to  move  vi^ithout  assistance  ? 

Caroline.  Oh  yjes ;  and  this  was  a  lever  of  the  se- 
cond order  (fig.  7.) ;  the  end  of  the  stick,  which  he 
thrusts  under  the  ball,  and  which  rests  on  the  ground, 
becomes  the  fulcrum  ;  the  ball  is  the  weight  to  be  moved 
and  the  power  his  hands  applied  to  the  other  end  of  the 
lever.  In  this  instance  there  is  an  immense  difference 
ih  the  length  of  the  arms  of  the  lever ;  for  the  weight 
is  almost  close  to  the  fulcrum. 

Mrs.  B,  And  the  advantage  gained  is  proportional 
to  this  difference.  Fishermen's  boats  are  by  levers  of 
this  description  raised  from  the  ground  to  be  launched 
into  the  sea,  by  means  of  slippery  pieces  of  board 
which  are  thrust  under  the  keel.  The  most  common  ex- 
ample that  we  have  of  levers  of  the  second  kind  is  in 
the  doors  of  our  apartments. 

Emily.  The  hinges  represent  the  fulcrum,  our  hands 
the  power  applied  to  the  other  end  of  the  lever;  but 
where  is  the  weight  to  be  moved  ? 

Mrs.  B.  The  door  is  the  weight,  and  it  consequently 
occupies  the  whole  of  the  space  between  the  power 
and  the  fulcrum.  Nutcrackers  are  double  levers  of 
this  kind  ;  the  hinge  is  the  fulcrum,  the  nut  the  resist- 
ance, and  the  hands  the  power. 

In  levers  of  the  third  kind  (fig.  8.),  the  fulcrum  is  a- 
gain  at  one  of  the  extremities,  the  weight  or  resistance 
at  the  other,  and  it  is  now  the  power  which  is  applied 
between  the  fulcrum  and  the  resistance. 

Emily.  The  fulcrum,  the  weight,  and  the  power, 
then,  each  in  their  turn,  occupy  some  part  of  the  midr 
die  of  the  lever  between  its  extremities.    But  in  this 


ON  THE  MECHANICAL  POWERS.  91 

third  kind  of  lever,  the  weight  being  farther  from  the 
centre  of  motion  than  the  power,  the  difficulty  of  rai- 
sing it  seems  increased  rather  than  diminished. 

Mrs.  B.  That  is  very  true  ;  a  lever  of  this  kind  is 
therefore  never  used,  unless  absolutely  necessary,  as  is 
the  case  in  lifting  up  a  ladder  perpendicularly  in  order 
to  place  it  against  the  wall ;  the  man  who  raises  it  can- 
not place  his  hands  on  the  upper  part  of  the  ladder,  the 
power,  therefore,  is  necessarily  placed  much  nearer 
the  fulcrum  than  the  w^eight. 

Caroline,  Yes,  the  hands  are  (he  power,  the  ground 
the  fulcrum,  and  the  upper  part  of  the  ladder  the 
weight. 

Mrs,  B,  Nature  employs  this  kind  of  lever  in  the 
structure  of  tlie  human  frame.  In  lifting  a  weight  with 
the  hand,  the  lower  part  of  the  arm  becomes  a  lever  of 
the  third  kind*;  the  elbow  is  the  fulcrum,  the  muscles  of 
the  fleshy  part  of  the  arm  the  power;  and  as  these  are 
nearer  to  the  elbow  than  the  hand,  it  is  necessary  that 
their  power  should  exceed  the  weight  to  be  raised. 

Emily,  Is  it  not  surprising  that  nature  should  have 
furnished  us  with  such  disadvantageous  levers  ^ 

Mrs.  B.  The  disadvantage,  in  respect  to  power,  is 
more  than  counterbalanced  by  the  convenience  resulting 
from  this  structure  of  the  arm  ;  and  it  is  no  doubt  that 
which  is  best  adapted  to  enable  it  to  perform  its  va- 
rious functions. 

We  have  dwelt  so  long  on  the  lever,  that  we  must  re- 
serve the  e-^amination  of  the  other  mechanical  powers 
to  our  next  interview. 


h,h.  hy  J.Y.Hunip7,reysT7fil.u1: 


CONVERSATION  V. 

CONTINUED. 


ON  THE  MECHANICAL  POWERS. 

or  THE  rULLF.T. — OF  THE  WHEEL  AND  AXLE.— OP  THE  INCLINED 
PLANE.— OF  THE  WEDGE.^ — OF  THE  SCREW. 


Mrs.  B. 

The  pulley  is  the  second  mechanical  power  we  are  to 
examine.     You,  both,  1  suppose,  have  seen  a  pulley  ? 

Caroline.  Yes,  frequently  :  it  is  a  circular  and  flat 
piece  of  wood  or  metal,  with  a  string  which  runs  in  a 
groove  round  it;  by  means  of  which,  a  weight  maybe 
pulled  up;  thus  pulleys  are  used  for  drawing  up  cur- 
tains. 

Mrs.  B.  Yes ;  but  in  that  instance  the  pulleys  are 
fixed,  and  do  not  increase  the  power  to  raise  the  weights, 
as  you  will  perceive  by  this  figure,  (plate  V.  fig.  1.)  Ob- 
serve that  the  fixed  pulley  is  on  the  same  principle  as 
the  lever  of  a  pair  of  scales,  in  which  the  fulcrum  F  being 
in  the  centre  of  gravity,  the  power  P  and  the  weight 
W,  are  equally  distant  from  it,  and  no  advantage  is 
gained. 


94       GN  THE  MECHANICAL  POWERS. 

Emily.  Certainly ;  if  P  represents  the  power  tm- 
ployed  to  raise  the  weight  W,  the  power  must  be  greater 
than  the  weight  in  order  to  move  it.  But  of  what  use 
then  are  pulleys  in  mechanics  ? 

Mrs.  B.  The  next  figure- represents  a  pulley  which 
is  not  fixed,  (fig.  2.)  and  thus  situated  you  will  perceive 
that  it  affords  us  mechanical  assistance.  In  order  to 
raise  the  weight  (W)  one  inch,  P,  the  power,  must  draw 
the  strings  B  and  C  one  inch  each  ;  the  whole  string 
is  therefore  shortened  two  inches,  while  the  weight  is 
raised  only  one. 

Emily.  That  I  understand :  if  P  drew  the  string 
but  one  inch,  the  weight  would  be  raised  only  half  an 
inch,  because  it  would  shorten  Jthe  strings  B  and  C  half 
an  inch  each,  and  consequently  the  pulley  with  the 
weight  attached  to  it,  can  be  raised  only  half  an  inch. 

Caroline,  I  am  ashamed  of  my  stupidity  ;  but  I  con- 
fess that  I  do  not  understand  this ;  it  appears  to  me 
that  the  weight  would  be  raised  as  much  as  the  string  is 
shortened  by  the  power. 

Mrs.  B,  I  will  endeavour  to  explain  it  more  clear- 
ly. I  fasten  this  string  to  a  chair  and  draw  it  towards 
me ;  I  have  now  shortened  the  string,  by  the  act  of 
drawing  it,  one  yard. 

Caroline.  And  the  chair,  as  I  supposed,  has  advan- 
ced one  yard. 

Mrs.  B.  This  exemplifies  the  nature  of  a  single 
fixed  pulley  only.  Now  unfasten  the  string,  and  re- 
place the  chair  where  it  stood  before.  In  order  to  re- 
present the  moveable  pulley,  we  must  draw  the  chair 
forwards  by  putting  the  string  round  it ;  one  end  of  the 
string  may  ^e  fastened  to  the  leg  of  the  table,  and  I  shall 


6N  THE  MECHANICAL  POWERS.  95 

draw  the  chair  by  the  other  end  of  the  string.  I  have 
again  shortened  the  string  one  yard  ;  how  much  has  the 
chair  advanced  ? 

Caroline.  I  now  understand  it ;  the  chair  represents 
the  weight  to  which  the  m^)veable  pulley  is  attached  ; 
and  it  is  very  clear  that  the  weight  can  be  drawn  only 
half  the  length  you  draw  the  string.  I  believe  the  cir- 
cumstance that  perplexed  me  was,  that  I  did  not  ob- 
serve the  difference  that  results  from  the  weight  being 
attached  to  the  pulley,  instead  of  being  fastened  to  the 
string,  as  is  the  case  in  the  fixed  [lulley. 

Emily,  J^ut  I  do  not  yet  understand  the  advantage 
of  pulleys ;  they  seem  to  me  to  increase  rather  thaa 
diminish  the  difficulty  of  raising  weights,  since  you 
must  draw  the  string  double  the  length  that  you  raise 
the  weight ;  whilst  with  a  single  pulley,  or  without  any 
pulley,  the  weight  is  raised  as  much  as  the  string  is 
shortened. 

Mrs,  B.  The  advantage  of  a  moveable  pulley  con- 
sists in  dividing  the  difficulty  ;  we  must  draw,  it  is  true, 
twice  the  length  of  the  string,  but  then  only  half  the 
strength  is  required  that  would  be  necessary  to  raise 
the  weight  without  the  assistance  of  a  moveable  pulley. 

Emily,  So  that  the  difficulty  is  overcome  in  the 
same  manner  as  it  would  be,  by  dividing  the  weight 
into  two  equal  parts,  and  raising  them  successively. 

Mrs,  B,  Exactly.  You  must  observe,  that  with  a 
moveable  pulley  the  velocity  of  the  power  is  double 
that  of  the  weight,  since  the  power  P  (fig.  2.)  moves 
two  inches,  whilst  the  weight  W  moves  one  inch  ;  there- 
fore the  power  need  not  be  more  thau  half  the  weight 
to  make  their  momen turns  equal. 


96  ON  THE  MECHANICAL  POWERS. 

Caroline.  Pulleys  act  then  on  the  same  principle  as 
the  lever,  the  deficiency  of  strength  oPthe  power  be- 
ing compensated  by  its  superior  velocity. 

Mrs.  B.  You  will  find,  that  all  mechanical  power 
is  founded  on  the  same  principle. 

Emily.  But  may  it  not  be  objected  to  pulleys,  that 
a  longer  time  is  required  to  raise  a  weight  by  their  aid 
than  without  it ;  for  what  you  gain  in  power  you  lose  in 
time  ? 

Mrs.  B.  That,  my  dear,  is  the  fundamental  law  in 
mechanics  :  it  is  the  case  with  th«  lever  as  well  as  the 
pulley ;  and  you  will  find  it  to  be  so  wit'.i  all  the  other 
mechanical  powers. 

Caroline,  I  do  not  see  any  advantage  in  the  mecha- 
nical powers  then,  if  what  we  gain  by  them  one  way  is 
lost  another. 

Mrs.  B.  Since  we  are  not  able  to  increase  our  na- 
tural strength,  is  not  that  science  of  wonderful  utility, 
by  means  of  which  we  may  reduce  the  resistance  or 
weii2;ht  of  any  body  to  the  level  of  our  strength  ?  This 
the  mechanical  powers  enable  us  to  accomplish,  by  di- 
viding the  resistance  of  a  body  into  parts  which  we  can 
successively  overcome.  It  is  true,  as  you  observe,  that 
it  requires  a  sacrifice  of  time  to  attain  this  end,  but 
you  must  be  sensible  how  very  advanta:^eously  it  is  ex-r 
changed  for  power :  the  utmost  exei  tiun  we  can  make 
adds  but  little  to  our  natural  strength,  whilst  we  have 
a  much  more  unlimited  command  of  time.  You  can 
now  understand,  that  the  greater  the  number  of  pulleys 
connected  by 'a  string,  the  more  easily  the  weight  is 
raised,  as  the  difEculty  is  divided  among  the  number  of 
strings,  or  rather  of  parts  into  which  the  string  is  di- 


ON  THE  MECHANICAL  POWERS.  97 

vided  by  the  pulleys.  Several  pulleys  thus  connected, 
form  what  is  called  a  system,  or  tackle  of  pulleys. 
(fig.  3.)  You  may  have  seen  them  suspended  from 
cranes  to  raise  goods  into  warehouses,  and  in  ships  to 
draw  up  the  sails. 

Emily,     But   since  a  fixed  pulley  affords  us  no  me*  . 
chanical  aid,  why  is  it  ever  used  ? 

Mrs,  B,  Though  it  does  not  increase  our  power,  it 
is  frequently  useful  for  altering  its  direction.  A  single 
pulley  enables  us  to  draw  up  a  curtain,  by  drawing 
down  the  string  connected  with  it;  and  we  should  be 
much  at  a  loss  to  accomplish  this  simple  operation  with- 
out its  assistance. 

Caroline.  There  would  certainly  be  some  difficulty  in 
ascending  to  the  head  of  the  curtain,  in  order  to  draw 
it  up.  Indeed,  I  now  recollect  having  seen  workmen 
raise  small  weights  by  this  means,  which  seemed  to 
answer  a  very  useful  purpose. 

Mrs.  B.  In  shipping,  both  the  advantages  of  an  in- 
crease of  power  and  a  change  of  direction,  by  means  of 
pulleys,  are  united  :  for  the  sails  are  raised  up  the 
masts  by  the  sailors  on  deck,  from  the  change  of  direc- 
tion which  the  pulley  effects,  and  the  labour  is  facilita- 
ted by  the  mechanical  power  of  a  combination  of  pul- 
leys. 

Emily.  But  the  pulleys  on  ship-board  do  not  appear 
to  me  to  be  united  in  the  manner  you  have  shown  us. 

Mrs.  B.  They  are,  I  believe,  generally  connected 
as  described  in  figure  4,  both  for  nautical,  and  a  varie- 
ty of  other  purposes  ;  but  in  whatever  manner  pul- 
leys are  connected  by  a  single  string,  the  mechanical 
power  is  the  same. 
t 


98  ON  TttE  MECHANICAL  POWERS. 

The  third  mechanical  power  is  the  wheel  and  axle. 
Let  us  suppose  (plate  VI.  fig.  5.)  the  weight  W  to  be  a 
bucket  of  water  in  a  well,  which  we  raise  by  winding 
the  rope,  to  which  it  is  attached,  round  the  axle ;  if 
this  be  done  without  a  wheel  to  turn  the  axle,  no 
mechanical  assistance  is  received.  The  axle  without  a 
wheel  is  as  impotent  as  a  single  fixed  pulley,  or  a  le- 
ver, whose  fulcrum  is  in  the  centre:  but  add  the  wheel 
to  the  axle,  and  you  will  immediately  find  the  bucket  is 
raised  with  much  less  difficulty.  The  velocity  of  the 
circumference  of  the  wheel  is  as  much  greater  than 
that  of  the  axle,  as  it  is  further  from  the  centre  of 
motion  :  for  the  wheel  describes  a  great  circle  in  the 
same  space  of  time  that  the  axle  describes  a  small 
one,  therefore  the  power  is  increased  in  the  same 
proportion  as  the  circumference  of  the  wheel  is 
greater  than  that  of  the  axle.  If  the  velocity  of  the 
wheel  is  twelve  times  greater  than  that  of  the  axle,  a 
power  nearly  twelve  times  less  than  the  weight  of 
the  bucket  would  be  able  to  raise  it. 

Emily.  The  axle  acts  the  part  of  the  shorter  arm 
of  the  lever,  the  wheel  that  of  the  longer  arm. 

Caroline,  In  raising  water,  there  is  commonly,  I  be- 
lieve, instead  of  a  wheel  attached  to  the  axle,  only  a 
crooked  handle,  which  answers  the  purpose  of  wind- 
in  j;-  the  rope  round  the  axle,  and  thus  raising  the 
bucket. 

Mrs.  B,  In  this  manner  (fig.  6.)  ;  now  if  you  observe 
the  dotted  circle  which  the  handle  describes  in  wind- 
ing up  the  rope,  you  will  perceive  that  the  branch  of 
the  handle  A,  which  is  united  to  the  axle,  represents 
the  spoke  of  a  wheel,  and  answers  the  purpose  of  an 


ON  THE  MECHANICAL  POWERS,  99 

entire  wheel ;  the  other  branch  B  affords  no  mechani- 
cal aid,  merely  serving  as  a  handle  to  turn  the  wheel. 

Wheels  are  a  very  essential  part  of  most  machines: 
they  are  employed  in  various  ways.;  but,  when  fixed  to 
the  axle,  their  mechanical  power  is  always  the  same ; 
that  is,  as  the  circumference  of  the  wheel  exceeds  that 
of  the  axle,  so  much  will  the  energy  of  the  power  be 
increased. 

Caroline,  Then  the  larger  the  wheel  the  greater  must 
be  its  effect. 

Mrs.  B.  Certainly.  If  you  have  ever  seen  anv  con- 
siderable mills  or  manufactures,  you  must  have  admired 
the  immense  wheel,  the  revolution  of  which  puts  the 
whole  of  the  machinery  into  motion  ;  and  though  so 
great  an  effect  is  produced  by  it,  a  horse  or  two  has 
su(ficient  power  to  turn  it  ;  sometimes  a  stream  of  wa- 
ter is  used  for  that  purpose,  but  of  late  years,  a  steam- 
engine  has  been  found  both  the  most  powerful  and  the 
most  convenient  mode  of  turning  the  wheel. 

Caroline.  Do  not  the  vanes  of  a  windmill  represent 
a  wheel,  Mrs.  B  ? 

Mrs.  B.  Yes  ;  and  in  this  instance  we  have  the  ad- 
vantage of  a  gratuitous  force,  the  wind,  to  turn  the 
wheel.  One  of  the  great  benefits  resulting  from  the  use 
of  machinery  is,  that  it  gives  us  a  sort  of  empire  over 
the  powers  of  nature,  and  enables  us  to  make  them 
perform  the  labour  which  would  otherwise  fall  to  the 
lot  of  man.  When  a  current  of  wind,  a  stream  of  wa- 
ter, or  the  expansive  force  of  steam,  performs  our 
task,  we  have  only  to  superintend  and  regulate  their 
operations. 


100  ON  THE  MECHANICAL  POWERS. 

The  fourth  mechanical  power  is  the  inclined  plane  ; 
this  is  nothing  more  than  a  slope,  or  declivity,  frequent- 
ly used  to  facilitate  the  drawing  up  of  weights.  It  is 
not  difficult  to  understand,  that  a  weight  may  much 
more  easily  be  drawn  up  a  slope  than  it  can  be  raised 
the  same  height  perpendicularly.  But  in  this,  as  well 
as  the  other  mechanical  powers,  the  facility  is  purcha- 
sed by  a  loss  of  time  (fig.  7.) ;  for  the  weight,  instead  of 
moving  directly  from  A  to  C,  must  move  from  B  to  C, 
and  as  the  length  of  the  plane  is  to  its  height,  so  much 
is  the  resistance  of  the  weight  diminished. 

Emily,  Yes ;  for  the  resistance,  instead  of  being 
confined  to  the  short  line  A  C,  is  spread  over  the  long 
line  B  C, 

Mrs.  B.  The  wedge,  which  is  the  next  mechanical 
power,  is  composed  of  two  inclined  planes  (fig.  8.)  :  you 
may  have  seen  wood-cutters  use  it  to  cleave  wood. 
The  resistance  consists  in  the  cohesive  attraction  of 
the  wood,  or  any  other  body  which  the  wedge  is  em- 
ployed to  separate ;  and  the  advantage  gained  by  this 
power  is  in  the  proportion  of  half  its  width  to  its  length ; 
for  while  the  wedge  forces  asunder  the  coherent  par- 
ticles of  the  wood  to  A  and  B,  it  penetrates  downwards 
as  far  as  C. 

Emily.  The  wedge,  then,  is  rather  a  compound  than 
a  distinct  mechanical  power,  since  it  is  composed  of 
two  inclined  planes. 

Mrs,  B.  It  is  so.  All  cutting  instruments  are 
constructed  upon  the  principle  of  the  inclined  plane, 
or  the  wedge :  those  that  have  bat  one  edge  sloped,  like 
the  chisel,  may  be  referred  to  the  inclined  plane ;  whilst 


ON  THE  MECHANICAL  POWERS.  101 

the  axe,  the  hatchet,  and  the  knife  (when  used  to  split 
asunder)  are  used  as  wedges. 

Caroline.  But  a  knife  cuts  best  when  it  is  drawn- 
acrosss  the  substance  it  is  to  divide.  Wc  use  it  thus 
in  cutting  meat,  we  do  not  chop  it  to  pieces. 

Mrs.  I],  The  reason  of  this  is,  that  the  edge  of  a 
knife  is  really  a  very  fine  saw,  and  therefore  acts  best 
when  used  like  that  iastru!nent. 

The  screv/,  wliich  is  tlie  last  mechanical  power,  is 
more  complicated  than  the  others.  You  will  see  by  this 
figure,  (fig.  9.)  that  it  is  composed  of  two  parts,  the 
screw  and  the  nut.  Tiie  screw  S  is  a  cylinder,  with  a 
spiral  protuberance  coiled  round  it,  called  the  thread; 
the  nut  N  is  perforated  to  contain  the  screw,  and  the 
inside  of  the  nut  has  a  spiral  groove  made  to  fit  the  spi- 
ral thread  of  the  screw. 

Caroline.  It  is  just  like  this  little  box,  the  lid  of 
which  screws  on  the  box  as  you  have  described ;  but 
what  is  this  handle  which  projects  from  the  nut. 

Mrs,  B,  It  is  a  lever,  which  is  attached  to  the  nut, 
without  which  the  screw  is  never  used  as  a  mechanical 
power :  the  nut  with  a  lever  L  attached  to  it,  is  com- 
monly called  a  winch.  The  power  of  the  screw,  com- 
plicated as  it  appears,  is  referable  to  one  of  the  most 
simple  of  the  mechanical  powers  ;  which  of  them  do 
you  think  it  is  ? 

Caroline.  In  appearance,  it  most  resembles  the 
livheel  and  axle. 

Mrs,  B.  The  lever,  it  is  true,  has  the  effect  of  a  wheel, 

as  it  is  the  means  by  which  you  wind  the  nut  round  ; 

but  the  lever  is  not  considered  as  composing  a  part  of 

the  screw,  though  it  is  true,  that  it  is  necessarily  at- 

j.  2 


102  ON  THE  MECHANICAL  POWERS. 

tached  to  it.  But  observe,  that  the  lever,  considered 
as  a  wheel,  is  not  fastened  to  the  axle  or  screw,  but 
moves  round  it,  and  in  so  doing,  the  nut  either  rises  or 
descends,  according  to  the  way  in  which  jou  turn  it. 

Emily.  The  spiral  thread  of  the  screw  resembles, 
I  think  an  inclined  plane :  it  is  a  sort  of  slope,  by 
means  of  which  the  nut  ascends  more  easily  than  it 
would  do  if  raised  perpendicularly ;  and  it  serves  to 
support  it  when  at  rest. 

Mrs,  B,  Very  well :  if  you  cut  a  slip  of  paper  in 
the  form  of  an  inclined  plane,  and  wind  it  round  your 
pencil,  which  will  represent  the  cylinder,  you  will  find 
that  it  makes  a  spiral  line,  corresponding  to  the  spiral 
protuberance  of  the  screw,  (fig.  10.) 

JEmily,  Very  true;  the  nut  then  ascends  an  inclined 
plane,  but  ascends  it  in  a  spiral,  instead  of  a  straight 
line:  the  closer  the  thread  of  the  screw,  the  more  easy 
the  ascent ;  it  is  like  having  shallow,  instead  of  steep 
steps  to  ascend. 

Mrs.  B.  Yes  ;  excepting  that  the  nut  takes  no  steps, 
it  gradually  winds  up  or  down;  then  observe,  that  the 
closer  the  threads  of  the  screw,  the  greater  the  num- 
ber of  revolutions  the  winch  must  make ;  so  that  we 
return  to  the  old  principle, — what  is  saved  in  power  is 
lost  in  time. 

Emily.  Cannot  the  power  of  the  screw  be  increased 
also,  by  lengthening  the  lever  attached  to  the  nut  ? 

Mrs.  B.  Certainly.  The  screw,  with  the  addition  of 
the  lever,  forms  a  very  powerful  machine,  employed 
either  for  compression  or  to  raise  heavy  weights.  It  is 
used  by  book-binders,  to  press  the  leaves  of  books  to- 


On  the  mechanical  powers,  103 

gether ;  it  is  used  also  in  cyder  and  wine  presses,  in 
coining,  and  for  a  variety  of  other  purposes. 

All  machines  are  composed  of  one  or  more  of  these 
six  me'^hanical  powers  we  have  examined  :  I  have  but 
one  more  remark  to  make  to  you  relative  to  them,  which 
is,  that  friction  in  a  considerable  degree  diminishes 
their  force,  allowance  must  therefore  always  be  made 
for  it,  in  the  construction  of  machinery. 

Caroline,  By  friction,  do  you  mean  one  part  of  the 
machine  rubbing  against  another  part  contiguous  it. 

Mi'S.  B,  Yes ;  friction  is  the  resistance  which  bodies 
meet  with  in  rubbing  against  each  other ;  there  is  no 
such  thing  as  perfect  smoothness  or  evenness  in  na- 
ture :  polished  metals,  though  they  wear  that  appear- 
ance, more  than  any  other  bodies,  are  far  from  really 
possessing  it ;  and  their  inequalities  may  frequently 
be  perceived  through  a  good  magnifying  glass.  When, 
therefore,  the  surfaces  of  the  two  bodies,  come  into 
contact,  the  prominent  parts  of  the  one  will  often  fall 
into  the  hollow  parts  of  the  other,  and  occasion  more 
or  less  resistance  to  motion. 

Caroline.  But  if  a  machine  is  made  of  polished  me- 
tal, as  a  watch  for  instance,  the  friction  must  be  very 
trifling  ? 

Mrs.  B.  In  proportion  as  the  surfaces  of  bodies  are 
well  polished,  the  friction  is  doubtless  diminished  ;  but 
it  is  always  considerable,  and  it  is  usually  computed  to 
destroy  one-third  of  the  power  of  a  machine.  Oil  or 
grease  is  used  to  lessen  friction  :  it  acts  as  a  polish  by 
filling  up  the  cavities  of  the  rubbing  surfaces,  and  thus 
making  them  slide  more  easily  over  each  other. 


104  @N  THE  MECHANICxVL  POWERS. 

Caroline,  Is  it  for  this  reason  that  wheels  are  greas- 
ed, and  the  locks  and  hinges  of  doors  oiled  ? 

Mrs,  B,  Yes;  in  these  instances  the  contact  of 
the  rubbing  surfaces  is  so  close,  and  the  rubbing  so  con- 
tinual, that  notwithstanding  their  being  polished  and 
oiled,  a  considerable  degree  of  friction  is  produced. 

There  are  two  kinds  of  friction  ;  the  one  occasioned 
by  the  sliding  of  the  flat  surface  of  a  body,  the  other 
bj  the  rolling  of  a  circular  body  ;  the  friction  resulting 
from  the  first  is  much  the  most  considerable,  for  great 
force  is  required  to  enable  the  sliding  body  to  overcome 
the  resistance  which  the  asperities  of  the  surfaces  in 
contact  oppose  to  its  motion,  and  it  must  be  either 
lifted  over,  or  breakthrough  them;  whilst,  in  the  other 
kind  of  friction,  the  rough  parts  roll  over  each  other 
with  comparative  facility;  hence  it  is,  that  wheels  are 
often  used  for  the  sole  purpose  of  diminishing  the  re- 
sistance of  friction. 

Emili}.  This  is  one  of  the  advantages  of  carriage- 
wheels;  is  it  not? 

Mrs,  B,  Yes;  and  the  larger  the  circumference  of 
the  wheel  the  more  readily  it  can  overcome  any  consi- 
derable obstacles,  such  as  stones,  on  inequalities  in  the 
road.  When,  in  descending  a  steep  hill,  we  fasten  one 
of  the  wheels,  we  decrease  the  velocity  of  the  carriage, 
by  increasing  the  friction. 

Caroline,  That  is  to  say,  by  converting  the  rolling 
friction  into  the  dragging  friction.  And  when  you  had 
casters  put  to  the  legs  of  the  table,  in  order  to  move  it 
more  easily,  you  changed  the  dragging  into  the  rolling 
friction. 


ON  THE  MECHANICAL  POWERS.  105 

Mrs,  B.  There  is  another  circumstance  which  we 
have  already  noticed,  as  diminishing  the  motion  of 
bodies,  and  which  greatly  affects  the  power  of  machines. 
This  is  the  resistance  of  the  medium,  in  which  a  ma- 
chine is  worked.  All  fluids,  whether  of  the  nature  of 
air,  or  of  water,  are  called  mediums;  and  their  resis- 
tance is  proportioned  to  their  density  ;  for  the  more  mat- 
ter a  body  contains,  the  greater  the  resistance  it  will 
oppose  to  the  motion  of  another  body  striking  against  it. 

Emily.  It  would  then  be  much  more  difficult  to 
work  a  machine  under  water  than  in  the  air  ? 

Mrs.  B.  Certainly,  if  a  machine  could  be  worked  in 
vacuo,  and  without  friction,  it  would  be  perfect ;  but  this 
is  unattainable  ;  a  considerable  reduction  of  power  must 
therefore  be  allowed  for  the  resistance  of  the  air. 

We  shall  here  conclude  our  observations  on  the  me- 
chanical powers.  At  our  next  meeting  I  shall  endea- 
vour to  give  you  an  explanation  of  the  motion  of  the 
heavenly  bodies. 


CONVERSATION  VL 


CAUSES  OF  THE  EARTH'S  ANNUAL  MOTION. 

OF  THE  PLANETS,  AND  THEIR  MOTION.   OF  THE  DIUKNAX 
MOTION  OF  THE  EARTH  AND  PLANETS. 


Caroline. 

I  AM  come  to  you  to-day  quite  elated  with  the  spirit 
©f  opposition,  Mrs.  B. ;  for  I  have  discovered  such  a 
powerful  objection  to  your  theory  of  attraction,  that  I 
doubt  whether  even  your  conjuror  Newton,  with  his  ma- 
gic wand  of  attraction,  will  be  able  to  dispel  it. 

Mrs.  B,  Well  my  dear,  pray  what  is  this  weighty 
objection  ? 

Caroline*  You  say  that  bodies  attract  in  proportion 
to  the  quantity  of  matter  they  contain,  now  we  all 
know  the  sun  to  be  much  larger  than  the  earth  :  why, 
fe  tlierefore,  does  it  not  attract  the  earth ;  you  will  not, 
I  suppose,  pretend  to  say  that  we  are  falling  towards 
the  sun  ? 

Emily,  However  plausible  your  objection  appears, 
Caroline,  I  think  you  place  too  much  reliance  upon  it : 


108  CAUSES  OF  THE 

when  any  one  has  ^iven  such  convincing  proofs  of  sa- 
gacity and  wisdom  as  Sir  Isaac  Newton,  when  we  find 
that  his  opinions  are  universally  received  and  adopted, 
is  it  to  be  expected  that  any  objection  we  can  advance 
should  overturn  them  ? 

Caroline,  Yet  I  confess  that  I  am  not  inclined  t© 
yield  implicit  faith  even  to  opinions  of  the  great  New- 
ton :  for  what  purpose  are  we  endowed  with  reason,  if 
we  are  denied  the  privilege  of  making  use  of  it,  by 
.  judging  for  ourselves  ? 

,Mrs,  B,  It  is  reason  itself  which  teaches  us,  that 
when  we,  novices  in  science,  start  objections  to  theories 
established  by  men  of  acknowledged  wisdom,  we  should 
be  diffident  rather  of  our  own  than  of  their  opinion.  I 
am  far  from  wishing  to  lay  the  least  restraint  on  your 
questions  ;  you  cannot  be  better  convinced  of  the  truth 
of  a  system,  than  by  finding  that  it  resists  all  your  at- 
tacks, but  I  would  advise  you  not  to  advance  your  ob- 
jections with  so  much  confidence,  in  order  that  the  dis- 
covery of  their  fallacy  may  be  attended  with  less  mor- 
tification. In  answer  to  that  you  have  just  proposed,  I 
can  only  say,  that  the  earth  really  is  attracted  by  the 
sun. 

Caroline,  Take  care  at  least  that  we  are  not  con- 
sumed by  him,  Mrs.  B. 

Mrs,  B,  We  are  m  no  danger ;  but  our  magiciau 
Newton,  as  you  are  pleased  t)  call  him,  cannot  extri- 
cate himself  from  this  difficulty  without  the  aid  of 
some  cabalistical  figures,  which  I  must  draw  for  him. 

Let  us  suppose  the  earth,  at  its  creation,  to  have 
been  projected  forwards  into  universal  space :  we 
know  that  if  no  obstacle  impeded  its  course  it  would 


Plate  ^i. 


Fuh.  by  J.Y.liujnci>li^e-ys  PJuliui^ 


EARTH'S  ANNUAL  MOTION.  109 

proceed  in  the  same  direction,  and  with  a  uniform 
velocity  for  ever.  In  fig.  I.  plate  6.,  A  represents  the 
earth,  and  S  the  sun.  We  shall  suppose  the  earth  to 
be  arrived  at  the  point  in  which  it  is  represented  in 
the  figure,  having  a  velocity  which  would  carry  it  on  to 
B  in  the  space  of  one  month  ;  whilst  the  sun's  attrac- 
tion would  bring  it  to  C  in  the  same  space  of  time.  Ob- 
serve that  the  two  forces  of  projection  and  attractioa 
do  not  act  in  opposition,  but  perpendicularly,  or  at  a 
right  angle  to  each  other.  Can  you  tell  me  now,  how 
the  earth  will  move  ^ 

EmiiU'  I  recollect  3^our  teaching  us  that  a  body  act- 
ed upon  by  two  forces  perpendicular  to  each  other  would 
piove  in  the  diagonal  of  a  parallelogram ;  if,  therefore, 
I  complete  the  parallelogram  by  drawing  the  lines  C  D, 
B  D,  the  earth  will  move  in  the  diagonal  A  D. 

Mrs.  B,  A  ball  struck  by  two  forces  acting  perpen- 
dicularly, to  each  other,  it  is  true,  moves  in  the  diagonal 
of  a  parallelogram ;  but  you  must  observe  th/it  the 
force  of  attraction  is  continually  acting  upon  our  ter- 
restrial ball,  and  producing  an  incessant  deviation  from 
its  course  in  a  right  line,  which  converts  it  into  that  of 
a  curve  line  ;  every  point  of  which  may  be  considered 
as  constituting  the  diagonal  of  an  infinitely  small  pa- 
rallelogram. 

Let  us  detain  the  earth  a  moment  at  the  point  D,  and 
iionsider  how  it  will  be  affected  by  trie  combined  action 
of  the  two  forces  in  its  new  situation.  It  still  retains 
its  tendency  to  fly  off  in  a  straight  line  ;  but  a  straight 
line  would  now  carry  it  away  to  F,  whilst  <he  sun  would 
attract  it  in  the  direction  D  S;  how  then  will  it  pro- 
ci^ed? 


110  CAUSES  OF  THE 

Emily.  It  will  go  on  in  a  curve  line,  in  a  direction 
between  that  of  the  two  forces. 

Mrs.  B,  In  order  to  know  exactly  what  course  the 
earth  will  follow,  draw  another  parallelograkh  similar 
to  the  first,  in  which  the  line  D  F  describes  the  force  of 
projection,  and  the  line  D  S,  that  of  attraction  ;  and  you 
will  find  that  the  earth  will  proceed  in  the  curve  line 

Caroline.  You  must  now  allow  nie  to  draw  a  pa- 
rallelogram, Mrs.  B.  Let  me  consider  in  what  direction 
will  the  force  of  projection  now  impel  the  earth. 

Mrs,  B.  First  draw  a  line  from  the  earth  to  the  sun 
representing  the  force  of  attraction  ;  then  describe  the 
force  of  projection  at  a  right  angle  to  it. 

Caroline.  The  earth  will  then  move  in  the  curve 
G  I,  of  the  parallelogram  G  H  I K. 

Mrs.  B.  You  recollect  that  a  body  acted  upon  by 
two  forces,  moves  thro'  gh  a  diagonal  in  the  same  time 
that  it  would  have  moved  through  one  of  the  sides  of  tie 
parallelogram,  were  it  acted  upon  by  one  force  only. 
The  earth  has  passed  through  the  diagonals  of  these 
three  parallelograms  in  the  space  of  three  months,  and 
has  performed  one  quarter  of  a  circle  ;  and  on  the  same 
principle  it  will  go  on  till  it  has  completed  the  whole  of 
the  circle.  It  will  then  recommence  a  course,  which  it 
has  pursued  ever  since  it  first  issued  from  the  hand  of 
its  Creator,  and  which  there  is  every  reason  to  suppose 
it  will  continue  to  follow,  as  long  as  it  remains  in  ex- 
istence. 

Emily.  What  a  grand  and  beautiful  effect  resulting 
from  so  simple  a  cause  ! 

Caroline.    It  affords  an  example,  on  a  magnificent 


EARTH'S  ANNUAL  MOTION.  Ill 

•scale,  of  the  circular  motion  winch  you  taught  us  in  me- 
cliauics.  llie  attraction  of  the  sun  is  the  centripetal 
force,  which  confines  the  earth  to  a  centre  ;  aiul  the  im- 
pjl.-se  of  projection  the  centrifugal  force,  which  impels 
toe  earth  to  quit  the  sun  and  fiy  off  in  a  tangent. 

MrH,  B.  Kxactlj  »o,  A  simple  mode  of  illustrating 
the  eill'ct  of  these  combined  forces  on  the  earth,  is  to 
cut  a  slip  of  card  in  the  form  of  a  right  angle,  (tig.  9. 
plate  VI.)  to  (lehcribe  a  small  circle  atthe  angular  point 
representing  the  earth,  and  to  fasten  the  exiremity  of 
onp  of  the  legs  of  the  angle  to  a  fixed  point,  which  we 
shall  consider  as  the  sun.  Thus  situated,  the  angle  will 
represent  both  the  centrifugal  and  centripetal  forces ; 
and  if  you  (iraw  it  round  the  fixed  point,  you  will  see 
how  the  direction  of  the  centrifugal  force  varies,  con- 
stantly forming  a  tangent  to  the  circle  in  which  the 
earth  moves,  as  it  is  constantly  at  a  ri^ht  angle  with  the 
centripetal  force. 

Emily,  The  earth  then,  gravitates  towards  the  sua 
without  the  slightest  danger  either  of  approaching  nearer 
or  receding  further  fn>m  it.  How  admirably  this  is 
contrived  !  If  the  two  forces  which  produce  this  cir- 
cular motion  had  not  been  so  accurately  adjusted,  one 
would  ultimately  have  prevailed  over  the  other,  and  we 
should  eitjier  have  approached  so  near  the  sun  as  to 
have  been  burnt,  or  have  receded  so  far  from  it  as  to 
have  been  frozen. 

Mrs.  B.  What  will  you  say,  my  dear,  when  I  tell 
you,  that  these  two  forces  are  not,  in  fact,  so  proportion- 
ed as  to  produce  circular  motion  in  the  earth  ? 

Caroline.  You  must  explain  to  us,  at  least,  in  what 
manner  we  avoid  the  threatened  destruction. 


112  CAUSES  OF  THE 

Mrs.  B,  Let  us  suppose  that  when  the  earth  is  at  A; 
(fig.  3.)  its  projectile  force  should  not  have  given  it  a 
velocity  sufficient  to  counterbalance  that  of  gravity,  so 
as  to  enable  these  powers  conjointly  to  carry  it  rottnd 
the  sun  in  a  circle;  the  earth,  instead  of  describing  the 
line  A  C,  as  in  the  former  figure,  will  approach  nearer 
the  sun  in  the  line  A  B. 

Ctroline.  Under  these  circumstances,  I  see  not  what 
is  to  prevent  our  approaching  nearer  and  nearer  the  sun 
till  we  fall  into  it :  for  its  attraction  increases  as  we  ad- 
vance towards  it,  and  produces  an  accelerated  velocity 
in  the  earth  which  increases  the  danger. 

Mrs,  B.  And  there  is  yet  another  danger,  of  which 
you  are  not  aware.  Observe,  that  as  the  earth  ap- 
proaches the  sun,  the  direction  of  its  projectile  force  is 
no  longer  perpendicular  to  that  of  attraction,  but  inclines 
more  nearly  to  it.  When  the  earth  reaches  that  part  of 
its  orbit  at  B,  the  force  of  projection  would  carry  it  to 
D,  which  brings  it  nearer  the  sun  instead  of  bearing  it 
away  from  it. 

Emily,  If,  then,  we  are  driven  by  one  power  and 
drawn  by  the  other  to  this  centre  of  destruction,  how  is 
It  possible  for  us  to  escape  ? 

Mrs,  B.  A  little  patience,  and  you  will  find  that  we 
are  not  without  resource.  The  earth  continues  ap- 
proaching the  sun  with  a  uniformly  increasing  accele- 
rated motion,  till  it  reaches  the  point  E  ;  in  what  di- 
rection will  the  projectile  force  now  impel  it  ? 

Emily,  In  the  direction  E  F.  Here  then  the  two 
forces  act  perpendicularly  to  each  other,  and  the  earth 
is  situated  just  as  it  was  in  the  preceding  figure ;  there- 
fore, from  this  point,  it  should  revolve  round  the  sun  ia 
a  circle. 


EARTH'S  ANNUAL  MOTION.  113 

Mrs,  B.  No,  all  the  circumstances  do  not  agree.  In 
motion  round  a  centre,  you  recollect  that  the  centrifugal 
force  increases  with  the  veloc  ity  of  the  body,  or  in 
other  words,  the  quicker  it  moves  the  stronger  is  its 
tendency  to  fly  oil*  in  a  right  line.  When  the  earth, 
therefore,  arrives  at  E,  its  accelerated  motion  will  have 
so  far  increased  its  velocity,  and  consequently  its  cen- 
trifugal force,  that  the  latter  will  prevail  over  the  force 
©f  attraction,  and  drag  the  earth  away  from  the  sun  till 
it  reaches  G. 

Caroline.  It  is  thus,  then,  that  we  escape  from  the 
dangerous  vicinity  of  the  sun  ;  and  in  proportion  as  we 
recede  from  it,  the  force  of  its  attraction,  and,  conse- 
quently, the  velocity  of  the  earth's  motion,  are  di- 
minished. 

Mrs,  B,  Yes.  From  G  the  direction  of  projection 
is  towards  H,  that  of  attraction  towards  S,  and  the 
earth  proceeds  between  them  with  a  uniformly  retarded 
motion,  till  it  has  completed  its  revolution.  Thus  you 
see,  that  the  earth  travels  round  the  sun,  not  in  a  circle, 
but  an  ellipsis,  of  which  the  sun  occupies  one  of  the 
foci  ;  and  that  in  its  course  the  earth  alternately  ap- 
proaches, and  recedes  from  it,  without  any  danger  of 
being  either  swallowed  up,  or  being  entirely  carried 
away  from  it. 

Caroline,  And  I  observe,  that  what  I  apprehended 
to  be  a  dangerous  irregularity,  is  the  means  by  which 
the  most  perfect  order  and  harmony  are  produced  ! 

Emily,  The  earth  travels,  then,  at  a  very  unequal 
rate,  its  velocity  being  accelerated  as  it  approaches  the 
sun,  and  retarded  as  it  recedes  from  it. 


114  CAUSES  OF  THE 

Mrs.  B.  It  is  mathematically  demonstrable,  that,  in 
moving  round  a  point  towards  which  it  is  attracted,  a 
.body  passes  over  equal  areas  in  equal  times.  The 
whole  of  the  space  contained  within  the  earth's  orbit, 
is,  in  fig.  4.,  divided  into  a  number  of  areas,  or  spaces, 
1,2,  3,4,  &c.  all  of  which  are  of  equal  dimensions, 
though  of  very  different  forms ;  some  of  them,  you  see, 
are  long  and  narrow,  others  broad  and  short :  but  they 
oacli  of  them  contain  an  equal  quantity  of  space.  An 
imaginary  line  drawn  from  the  centre  of  the  earth  to 
that  of  the  sun,  and  keeping  pace  with  the  earth  in  its 
revolution,  passes  over  equal  areas  in  equal  times; 
that  is  to  say,  if  it  is  a  month  going  from  A  to  B,  it 
will  be  a  month  going  from  B  to  C,  and  another  fromC 
to  E,  and  so  on. 

Caroline,  What  long  journeys  the  earth  has  to  per- 
form in  the  course  of  a  month,  in  one  part  of  her  orbit, 
and  how  short  they  are  in  the  other  part ! 

Mrs,  B,  The  inequality  is  not  so  considerable  as 
appears  in  this  figure ;  for  the  earth's  orbit  is  not  so 
eccentric  as  it  is  there  described  ;  and  in  reality,  dif- 
fers but  little  from  a  circle  :  that  part  of  the  earth's  or- 
bit nearest  the  sun  is  called  its  perihelion,  that  part 
most  distant  from  the  sun  its  aphelion^  and  the  earth 
is  above  three  millions  of  miles  nearer  the  sun  at  its 
perihelion  than  at  its  aphelion. 

Emily.  I  think  I  can  trace  a  consequence  from 
these  different  situations  of  the  earth ;  is  it  not  the 
cause  of  summer  and  winter  ? 

Mrs.  B.  On  the  contrary ;  during  the  height  of 
summer,  the  earth  is  in  that  part  of  its  orbit  which  is 


EARTH'S  ANNUAL  MOTION.  115 

most  distant  from  the  sun,  and  it  is  during  the  severity 
of  winter,  that  it  approaches  nearest  to  it. 

Emily,  That  is  very  extraordinary  ;  and  how  then 
do  you  account  for  the  heat  beii%  greatest,  when  we 
are  most  distant  from  the  sun  ? 

Mrs.  B.  The  difference  of  the  earth's  dist-'^ce  from 
the  sun  in  summer  and  winter,  when  r^mpared  with 
its  total  distance  from  the  sun,  is  ^^^  inconsiderable. 
The  earth,  it  is  true,  is  above  ^'*»'ee  millions  of  miles 
nearer  the  sun  in  winter  tK^n  in  summer;  but  that  dis- 
tance, however  great  '^^  at  first  appears,  sinks  into  in- 
significance in  co^^iparison  of  95  millions  of  miles, 
which  is  our  me^n  distance  from  the  sun.  The  change 
of  tempera^re,  arising  from  this  diiference,  would 
scarcely  be  sensible;  were  it  not  completely  over- 
po^vered  by  other  causes  which  produce  the  variations 
of  the  seasons;  but  these  I  shall  defer  explaining,  till 
we  have  made  some  further  observations  on  the  hea- 
venly bodies. 

Caroline.  And  should  not  the  sun  appear  smaller  ia 
summer,  when  it  is  so  much  further  from  us  ^ 

Mrs.  B.  It  actually  does,  when  accurately  measu- 
red ;  but  the  apparent  difterence  in  size,  is  I  believe, 
not  perceptible  to  the  naked  eye. 

Eraily.  Then,  since  the  earth  moves  with  greatest 
velocity  in  that  part  of  its  orbit  nearest  the  sun,  it  must 
have  completed  its  journey  through  one  half  of  its  orbit 
in  a  shorter  time  than  the  other  half? 

Mrs.  B.  Yes,  it  is  about  seven  days  longer  per- 
forming the  summer-half  of  its  orbit,  than  the  winter- 
-half. 

The  revolution  of  all  the  planets  round  the  sun  is  the 


116  CAUSES  OF  THE 

result  of  the  same  causes,  and  is  performed  in  the  same 
manner  as  that  of  the  earth. 

Caroline.    Praj  what  are  the  planets  ? 

Mrs.  B.  They  are  those  celestial  bodies,  which  re- 
'^Ive  like  our  earth  about  the  sun ;  they  are  supposed 
to  rest,a.>i3jg  the  earth  also  in  many  other  respects ;  and 
we  are  lea  Sy  analogy  to  suppose  them  to  be  inhabited 
worlds. 

Caroline.  I  have  hv/^j-d  so  ;  but  do  you  not  think  such 
an  opinion  too  great  a  strt>ch  of  the  imagination  ? 

Mrs.  B.  Some  of  the  plai-^ts  are  proved  to  be  lar- 
ger than  the  earth ;  it  is  only  tht;^'  immense  distance 
from  us,  which  renders  their  appare^^t  dimensions  so 
small.  Now  if  we  consider  them  as  en^^rmous  globes, 
instead  of  small  twinkling  spots,  we  shall  be  led  to 
suppose,  that  the  Almighty  would  not  have  created 
them  merely  for  the  purpose  of  giving  us  a  little  light 
in  the  night,  as  it  was  formerly  imagined,  and  we  should 
find  it  more  consistent  with  our  ideas  of  the  Divine 
wisdom  and  beneficence  to  suppose  that  these  celestial 
bodies,  shpuld  be  created  for  the  habitation  of  beings, 
who  are,  like  us,  blessed  by  his  providence.  Both  in  a 
moral  as  well  as  a  physical  point  of  view,  it  appears  to 
me  more  rational  to  consider  the  planets  as  worlds  re- 
volving round  the  sun  ;  and  the  fixed  stars  as  other 
suns,  each  of  them  attended  by  their  respective  system 
of  planets,  to  which  they  impart  their  influence.  We 
have  brought  our  telescopes  to  such  a  degree  of  perfec- 
tion, that  from  the  appearances  which  the  moon  exhibits 
when  seen  through  them,  we  have  very  good  reason  to 
conclude,  that  it  is  a  habitable  globe,  for  though  it  is  true, 
that  we  cannot  discern  its  towns  and  people,  we  can 


EARTH'S  ANNUAL  MOTION,  117 

plainly  perceive  its  mountains  and  valleys ;  and  some 
astronomers  have  gone  so  far  as  to  imagine  they  disco- 
vered volcanos. 

Emily,  If  the  fixed  stars  are  suns,  with  planets  re- 
volving round  them,  why  should  we  not  see  those  pla- 
nets as  well  as  their  suns? 

Mrs,  B,  In  the  first  place,  we  conclude  that  the 
planets  of  other  systems,  (like  those  of  our  own,)  ar6 
much  smaller  than  the  suns  whi.'h  give  them  light; 
therefore  at  so  great  a  distance  as  to  make  the  suns 
appear  like  fixed  stars,  the  planets  would  be  quite  in- 
visible. Secondly,  the  light  of  the  planets  being  only 
reflected  light,  is  much  more  feeble  than  that  of  the 
fixed  stars.  There  is  exactly  the  same  difference  as  be- 
tween the  light  of  the  sun  and  that  of  the  moon ;  the 
first  being  a  fixed  star,  the  second  a  planet. 

Emily,  But  if  the  planets  are  worlds  like  our  earth, 
ihey  are  dark  bodies  ;  and  instead  of  shining  by  night, 
we  should  see  them  only  by  day-light.  And  why  do 
we  not  see  the  fixed  stars  also  by  day-light  ? 

Mrs,  B,  Both  for  the  same  reason  ;— -their  light  is 
so  faint,  compared  to  that  of  our  sun  reflected  by  the 
atmosphere,  that  it  is  entirely  effaced  by  it :  the  light 
emitted  by  the  fixed  stars  may  probably  be  as  strong 
as  that  of  our  sun,  at  an  equal  distance  ;  but  being  so 
much  more  remote,  it  is  diffused  over  a  gi  eater  space, 
and  is  consequently  proportionally  weakened. 

Caroline,  True;  I  can  see  much  better  by  the  light 
of  a  candle  that  is  near  me,  than  by  that  of  one  at  a 
great  distance.  But  I  do  not  understand  what  makes 
ihe  planets  shine  } 

Mrs.  B,  What  is  that  makes  the  steel  buttons  on 
your  brother's  coat  shine  ? 


118  CAUSES  OP  THE 

Caroline.  The  sun.  But  if  it  was  the  sun  which 
made  the  planets  shine,  we  should  see  them  in  tliC 
daj-time  when  the  sun  shown  upon  them;  or  if  the 
faintness  of  their  light  prevented  our  seeing  th  in  in 
the  day,  we  should  not  see  them  at  all,  for  the  sun 
cannot  shine  upon  them  in  ihe  nigbt. 

Mrs.  B.  There  you  are  in  error.  But  in  order  to 
explain  this  to  you,  I  must  first  make  you  acquainted 
with  the  various  motions  of  the  planets. 

You  know,  that  according  to  the  laws  of  attraction, 
the  planets  belonging  to  our  system  all  gravitate  tow- 
ards the  sun  ;  and  that  this  force,  combined  with  that 
©f  projection,  will,  occasion  their  revolution  round  the 
aun,  in  orbits  more  or  less  elliptical,  according  to  the 
proportion  which  these  two  forces  bear  to  each  other. 

But  the  planets  have  also  another  motion :  they  re- 
volve upon  their  axes.  The  axis  of  a  planet  is  an  im- 
aginary line  which  passes  through  its  centre,  and  on 
which  it  turns  ;  and  it  is  this  motion  which  produces  day 
and  night.  With  that  side  of  the  planet  facing  the  sua 
it  is  day  ;  and  with  the  opposite  side,  which  remains  in 
darkness  it  is  night.  Our  earth,  which  we  consider  as  a 
planet  is  24  hours  in  performing  one  revolution  on  its 
axis  ;  in  that  period  of  time,  therefore,  we  have  a  day 
and  a  night;  hence  this  revolution  is  called  the  earth's 
diurnal  or  daily  motion  ;  and  it  is  this  revolution  of  the 
earth  from  west  to  east  which  produces  an  apparent  mo- 
tion of  the  sun,  moon,  and  stars  in  a  contrary  direction. 

Let  us  now  suppose  ourselves  to  be  beings  indepen- 
dent of  any  planet,  travelling  in  the  skies,  and  looking 
upon  tlie  earth  in  the  same  point  of  view  as  upon  the 
other  planets. 


EARTH'S  ANNUAL  MOTION,  119 

Caroline.  It  is  not  tiattering  to  us,  its  inhabitants, 
to  see  it  make  so  insignificant  an  appearanc  e. 

Mrs.  B.  To  those  who  are  accustomed  to  contem- 
template  it  in  this  light,  it  never  appears  more  glorious. 
We  are  taught  by  science  to  distrust  appearances :  and 
instead  of  considering  the  planets  as  little  stars,  we 
look  upon  them  either  as  brilliant  suns  or  habitable 
worlds,  and  we  consider  the  whole  together  as  forming 
one  vast  and  magnificent  system,  worthy  of  the  Divine 
hand  by  which  it  was  created. 

Emily.  I  can  scarceh  conceive  the  idea  of  this  im- 
mensity of  creation  ;  it  seems  too  sublime  for  our  im- 
agination : — and  to  think  that  the  goodness  of  Provi- 
dence extends  over  millions  of  worlds  throughout  a 
boundless  universe — Ah  !  Mrs.  B.,  't  is  we  only  who  be- 
come trifling  and  insignificant  beings  in  so  magnificent 
a  creation  ! 

Mrs.  B.  This  idea  should  teach  us  humility,  but 
without  producing  despontUncy.  The  same  Almighty 
hand  which  guides  these  countit«s  worlds  in  their  unde- 
viating  course,  conducts  with  equal  pc^rfection  the  blood 
as  it  circulates  through  the  veins  of  a  ftj,  and  opens 
the  eye  of  the  insect  to  behold  His  wonders.  Notwith. 
standing  this  immense  scale  of  creation,  therefore,  we 
need  not  fear  to  be  disregarded  or  forgotten. 

But  to  return  to  our  station  in  the  skies.  We  were  if 
you  recollect,  viewing  the  earth  at  a  great  distance,  ia 
appearance  a  little  star,  one  side  illuminated  by  the  sun. 
the  other  in  obscurity.  But  would  you  believe  it,  Ca- 
roline, many  of  the  inhabitants  of  this  Uttle  star  ima- 
gine that  when  that  part  which  they  inhabit  is  turned 
from  the  sun,  darkness  prevails  Uiroughout  the  universe 


120  CAUSES  OP  THE 

merely  because  it  is  night  with  them  ;  whilst,  in  realitj 
the  sun  never  ceases  to  shine  upon  every  planet.  When 
therefore,  these  little  ignorant  beings  look  around  them 
during  their  night,  and  behold  all  the  stars  shining, 
they  cannot  imagine  why  the  planets,  which  are  dark 
bodies,  should  shine,  concluding,  that  since  the  sun 
does  not  illumine  themselves,  the  whole  universe  must 
be  in  darkness. 

Caroline.  I  confess  that  I  was  one  of  these  igno- 
rant people  ;  but  I  am  now  very  sensible  of  the  absur- 
dity of  such  an  idea.  To  the  inhabitants  of  the  other 
planets,  then,  we  must  appear  as  a  little  star? 

Mrs,  B,  Yes,  to  those  which  revolve  round  our  sun ; 
for  since  those  which  may  belong  to  other  systems  (and 
whose  existence  is  only  hypothetical,)  are  invisible  to 
tis,  it  is  probable,  that  we  also  are  invisible  to  them. 

Emily.  But  they  may  see  our  sun  as  we  do  theirs, 
in  appearance  a  fixed  star  ? 

Mrs.  B.  No  doubt ;  if  iyte^  beings  who  inhabit  those 
planets  are  endowed  w^^A  senses  similar  to  ours.  By 
the  same  rule,  wp  must  appear  as  a  moon,  to  the  in- 
habitants of  ^ur  moon ;  but  on  a  larger  scale,  as  the 
srai  face  of  the  earth  is  about  thirteen  times  as  large  as 
that  of  the  moon. 

Emily.  The  moon,  Mrs.  B.,  appears  to  move  in  a 
different  direction,  and  in  a  different  manner  from  the 
stars  ? 

Mrs.  B.  I  shall  defer  the  explanation  of  the  motion 
of  the  moon,  till  our  next  interview,  as  it  would  prolong 
oj^  present  lesson  too  much. 


CONVERSATION  VII. 


ON  THE  PLANETS. 

OF    THE    SATELLITES    OR    MOONS. GRAVITY     DIMINISHTES     AS    THE 

SaUARE  OF  THE  DISTANCE. OF  THE  SOLAR  SYSTEM. OF  COM- 
ETS.  CONSTELLATIONS,  SIGNS  OF  THE  ZODIAC. — OF  COPERNI- 
CUS,   NEWTON,    he. 


Mrs.  B. 

The  planets  are  distinguished  into  primary  and  se- 
condary. Those  which  revolve  immediately  about  the 
sun  are'  called  primary.  Many  of  these  are  attended 
in  their  course  by  smaller  planets,  which  revolve  round 
them  :  these  are  called  secondary  planets,  satellites,  or 
moons.  Such  is  our  moon  which  accompanies  the  earth, 
and  is  carried  with  it  round  the  sun. 

Emily,  How  then  can  you  reconcile  the  motion  of  the 
secondary  planets  to  the  laws  of  gravitation  -,  for  the 
sun  is  much  larger  than  any  of  the  primary  planets; 
and  is  not  the  power  of  gravity  proportional  to  the 
quantity  of  matter  ? 


122  ON  THE  PLANETS, 

Caroline,  Perhaps  the  sun,  though  much  larger 
may  be  less  dense  than  the  planets.  Fire  you  know  is 
very  light,  and  it  may  contain  but  little  matter,  though 
of  great  magnitude. 

Mrs,  B.  We  do  not  know  of  what  kind  of  matter  the 
sun  is  made ;  but  we  may  be  certain,  that  since  it  is 
the  general  centre  of  attraction  of  our  system  of  plan- 
ets, it  must  be  the  body  which  contains  the  greatest 
quantity  of  matter  in  that  system.  ^ 

You  must  recollect,  that  the  force  of  attraction  is 
not  only  proportional  to  the  q*iantity  of  matter,  but  to 
the  degree  of  proximity  of  the  attractive  body :  this 
power  is  weakened  by  being  diffused,  and  diminishes 
as  the  squares  of  the  distances  increase.  The  square 
is  the  product  of  a  number  multiplied  by  itself;  so  that 
a  planet  situated  at  twice  the  distance  at  which  we  are 
from  the  sun  would  gravitate  four  times  less  than  we 
do ;  for  the  product  of  two  multiplied  by  itself  is  four. 

Caroline.  Then  the  more  distant  planets  move 
slower  in  their  orbits  ;  for  their  projectile  force  must  be 
proportioned  to  that  of  attraction  ?  But  I  do  not  see 
how  this  accounts  for  the  motion  of  the  secondary 
round  the  primary  planets,  in  preference  to  ^'le  sun  ? 

Emily.  Is  it  not  because  the  vicinity  of  the  prima- 
ry planets  renders  their  attraction  stronger  than  that  of 
the  sun  ? 

Mrs,  B,  Exactly  so.  But  since  the  attraction  be" 
tween  bodies  is  mutual,  the  primary  planets  are  also 
attracted  by  the  satellites,  which  revolve  round  them. 
The  moon  attracts. the  earth,  as  well  as  the  earth  the 
moon ;  but  as  the  latter  is  the  smaller  body,  her  at- 
traction is  proportionally  less ;  therefore  neither  the 


ON  THE  PLANETS.  123 

earth  revolves  round  the  moon,  nor  the  moon  round 
the  earth  ;  but  they  both  revolve  round  a  point,  which 
is  their  common  centre  of  gravity,  and  which  is  as 
much  nearer  the  earth  than  the  moon,  as  the  gravity  of 
the  former  exceeds  that  of  the  latter. 

Emily,  Yes,  I  recollect  your  saying,  that  if  two  bodies 
were  fastened  together  by  a  wire  or  bai',  their  common 
centre  of  gravity  would  be  in  the  middle  of  the  bar,  pro- 
vided the  bodies  were  of  equal  weight;  and  if  they  dif- 
fered in  weight,  it  would  be  nearer  the  larger  body.  If 
then  the  earth  and  moon  had  no  projectile  force  which 
prevented  their  mutual  attraction  from  bringing  them 
together,  they  would  meet  at  their  common  centre  of 
gravity. 

Caroline*  The  earth  then  has  a  great  variety  of  mo- 
tions, it  revolves  round  the  sun,  upon  its  axis,  and  round 
the  point  towards  which  the  moon  attracts  it. 

Mrs,  B.  Just  so ;  and  this  is  the  case  with  every  pla- 
net which  is  attended  by  satellites.  The  complicated 
effect  of  this  variety  of  motions,  produces  certain  irre- 
gularities, which,  however,  it  Is  not  necessary  to  notice 
at  present. 

The  planetb  act  on  the  sun  in  the  same  manner  as  they 
are  themselves  acted  on  by  their  satellites ;  for  attraction, 
you  must  remember,  is  always  mutual ;  but  the  gravity 
of  the  planets  (even  when  taken  collectively)  is  so  tri- 
fling compared  with  that  of  the  sun,  that  they  do  not 
cause  the  latter  to  niove  so  much  as  one  half  of  his  di- 
ameter. The  planets  do  not,  therefore,  revolve  round  the 
centre  of  the  sun,  but  round  a  point  at  a  small  distance 
from  its  centre,  about  which  the  sun  also  revolves. 

Emily,    I  thought  the  sun  had  no  motion  ? 


1?  W^' 

154  ON  THE  PLAN1ETS. 

Mrs,  B.  You  were  mistaken  ;  for  besides  that  which 
I  have  just  mentioned,  which  is  indeed  very  inconsidera- 
ble, he  revolves  on  his  axis;  this  motion  is  ascertained  by 
observing  certain  spots  which  disappear,  and  reappear 
regularly  at  stated  times. 

Caroline.  A  planet  has  frequently  been  pointed  out 
to  me  in  the  heavens  ;  but  I  could  not  perceive  that  its 
motion  differed  from  that  of  t]ie  fixed  stars,  which  only 
appear  to  move. 

Mrs,  B,  The  great  distance  of  the  planets  renders 
their  motion  apparently  so  slow,  that  the  eye  is  not  sen- 
sible oftlieir  progress  in  their  orbit,  unless  we  watch 
them  for  some  considerable  length  of  time  :  in  different 
seasons  they  appear  in  different  parts  of  the  heavens. 
The  most  accurate  idea  1  can  give  you  of  the  situation 
and  inotion  of  the  planets,  will  be  by  the  examination 
of  this  diagram,  (plate  yil,  fig.  1.)  representing  the  so- 
lar system,  in  which  you  will  findevery  planet  with  its 
orbit  delineated. 

Emily,  But  the  orbits  here  are  all  circular,  and  you 
said  that  they  were  elliptical.  The  planets  appear  too, 
to  be  moving  round  the  centre  of  ihe  sun;  whilst  you 
told  us  that  they  moved  round  a  point  at  a  little  dis- 
tance from  thence. 

Mrs,  B,  The  orbits  of  the  planets  are  so  nearly 
circular  and  i\\Q,  common  centre  of  gravity  of  the  solar 
system  so  near  the  centre  of  the  sun,  that  these  devia- 
tions are  scarcely  worth  observing.  The  dimensions  of 
the  planets,  in  tiieir  true  proportions,  you  will  find  de- 
lineated in  fig.  2. 

Mercury  is  the  planet  nearest  the  sun  ;  his  orbit  is 
consequently  contained  within  ours  ;  but  his  vicinity 


Fiq.  1. 


F-U) .  2. 


Enrtii  /         ^\ 


O      O      O      (        ]""-'-"'^ 


r-uh.  hy.J.rJhjj,i^?In::ys  r/n/.ulf 


% 


ON  THE  PLANETS.  125 

to  the  sun,  occasions  his  being  nearly  lost  in  the  brilli- 
ancy of  his  rays ;  and  when  we  see  the  sun,  he  is  so 
dazzling,  that  very  accurate  observations  cannot  be 
made  upon  Mercury.  He  performs  his  revolution  round 
the  sun  in  about  87  days,  which  is  consequently  the 
length  of  his  year.  The  time  of  his  rotation  on  his 
axis  is  not  known  ;  his  distance  from  the  sun  is  comput- 
ed to  be  37  millions  of  miles,  and  his  diameter  3180 
miles.  The  heat  of  this  planet  is  so  great,  that  water 
cannot  exist  there,  but  in  a  state  of  vapour,  and  metals 
would  be  liquified. 

Car 0 tine.    Oh,  what  a  dreadful  climate  ! 

Mrs,  B,  Though  we  could  not  live  there,  it  may  be 
perfectly  adapted  to  other  beings  destined  to  inhabit  it. 

Venus,  the  next  in  the  order  of  planets,  is  68  mil- 
lions of  miles  from  the  sun :  she  revolves  about  her 
axis  in  23  hours  and  21  minutes,  and  goes  round  the  sun 
in  244  days  17  hours.  The  orbit  of  Venus  is  also  with* 
in  ours  ;  during  one  half  of  her  course  in  it,  we  see  her 
before  sun-rise,  and  she  is  called  the  morning  star ;  in 
the  other  part  of  her  orbit,  she  rises  later  than  the  sun. 

Caroline.  In  that  case,  we  cannot  see  her,  for  she 
must  rise  in  the  day  time  ? 

JSIrs.  B.  True ;  but  when  she  rises  later  than  the 
sun,  she  also  sets  later  ;  so  that  we  perceive  her  ap- 
proaching the  horizon  after  sun-set:  she  is  then  called 
Hesperus,  or  the  evening  star.  Do  you  recollect  those 
beautiful  lines  of  Milton  : 

Now  came  still  evening  on,  and  twilight  gray 
Had  in  her  sober  livery  all  things  clad ; 
Silence  accompanied ;  for  beast  and  bird. 
They  to  their  grassy  couch,  these  to  their  nests 

L.2 


126  ON  THE  PLANETS. 

Were  slunk,  all  but  the  wakeful-nightingale ; 
She  all  night  long  her  amorous  descant  sung; 
Silence  was  pleas'd :  now  glow'd  the  fii-mamen' 
"With  living  saphirs  :  Hesperus,  that  led    ~ 
The  starry  host,  rode  brightest,  till  the  moon 
Rising  in  clouded  majesty,  at  length 
Apparent  queen  unveiPd  her  peerless  light,- 
And  o'er  the  dark  her  silver  mantle  threw. 

The  planet  next  to  Venus  is  the  Earth,  of  which  we 
we  shall  soon  speak  at  full  length.  At  present  i  shall 
only  observe,  that  we  are  95  millions  of  miles  distant 
from  the  sun,  that  we  perform  our  annual  revolution  in 
365  days  5  hours  and  49  minutes ;  and  are  attended  in 
our  course  by  a  single* moon. 

Next  follows  Mars.  He  can  never  come  between  us 
and  the  sun,  like  Mercury  and  Venus  ;  his  motion  is, 
however,  very  perceptible,  as  he  may  be  traced  to  dif- 
ferent situations  in  the  heavens  ;  his  distance  from  the 
sun  is  144  millions  of  miles  ;  he  turns  round  his  axis 
in  24  hours  and  39  minutes  ;  and  he  performs  his  an- 
nual revolution,  in  about  687  of  our.  days  :  his  diameter 
is  4120  miles.  Then  follow  four  very  small  planets, 
Juno,  Ceres,  Pallas,  and  Vesta,  which  have  been  recent- 
ly discovered,  but  whose  dimensions  and  distances  from 
the  sun  have  not  been  very  accurately  ascertained. 

Jupiter  is  next  in  order:  this  is  the  largest  of  all 
the  planets.  He  is  about  490  millions  of  miles  from 
the  sun,  and  completes  his  annual  period  in  nearly  12  of 
our  years.  He  turns  round  his  axis  in  about  ten  hours. 
He  is  above  1200  times  as  big  as  our  earth ;  his  diame- 
ter being  86,000  miles.  The  respective  proportions  o^ 
the-  planets  cannot,  therefore,  you  see,  be  conveniently 
delineated  in  a  diagram.  He  is  attended  by  four  moon^. 


ON  THE  PLANETS.  127 

The  next  planet  is  Saturn,  whose  distance  from  the 
sun  is  about  900  millions  of  miles  ;  his  diurnal  rotation 
is  performed  in  10  hours  and  a  quarter : — his  annual 
revolution  in  nearly  30  of  our  years.  His  diameter 
is  79,000  miles.  This  planet  is  surrounded  by  a  lurni- 
nous  ring,  the  nature  of  which,  astronomers  are  much 
at  a  loss  to  conjecture ;  he  has  seven  moons.  Lastly, 
we  observe  the  Georgium  Sidus,  discovered  by  Dr. 
Herschel,  and  which  is  attended  by  six  moons. 

Caroline,  How  charming  it  must  be  in  the  distant 
planets,  to  see  several  moons  shining  at  the  same  time ; 
I  think  I  should  like  to  be  an  inhabitant  of  Jupiter  or 
Saturn. 

Mrs.  B,  Not  long,  1  believe#  Consider  what  ex- 
treme cold  must  prevail  in  a  planet,  situated  as  Saturn 
is,  at  nearly  ten  times  the  distance  at  which  we  are 
from  the  sun.  Tiien  his  numerous  moons  are  far  from 
making  so  splendid  an  appearance  as  ours ;  for  they 
can  reflect  only  the  light  which  they  receive  from  the 
sun ;  and  both  light  and  heat  decrease  in  the  same  ra- 
tio or  proportion  to  the  distances  as  gravity.  Can  you 
tell  me  now  how  much  more  light  we  enjoy  than  Saturn. 

Caroline,  The  square  of  ten,  is  a  hundred  ;  there- 
fore, Saturn  has  a  hundred  times  less — or  to  answer 
.  your  question  exactly,  we  have  an  hundred  times  more 
light  and  heat  than  Saturn  — this  tertainly  does  not  in- 
crease my  wish  to  become  one  of  the  poor  wretches 
who  inhabit  that  planet. 

Mrs,  B,  May  not  the  inhabitants  of  Mercury,  with 
equal  plausibility,  pity  us,  for  the  insupportable  coldness 
of  our  situation  ;  and  those  of  Jupiter  and  Saturn  for 
-otir  intolerable  heat?     The  Almighty  Power  which 


128  ON  THE  PLANETSK 

created  these  planets,  and  placed. them  in  their  several 
orbits,  has  no  doubt  peopled  them  with  beings  whose 
bodies  are  adapted  to  the  various  temperatures  and  ele- 
ments in  which  thej  are  situated.  If  we  judge  from 
the  analogy  of  our  own  earth,  or  from  that  of  the  great 
and  universal  beneficence  of  Providence,  we  must  con- 
clude this  to  be  the  case. 

Caroline.  Are  not  comets  also  supposed  to  be  planets? 

Mrs,  B.  Yes,  they  are ;  for  by  the  re-appearance  of 
some  of  them,  at  stated  times,  they  are  known  to  re- 
volve round  the  sun,  but  in  orbits  so  extremely  excen- 
tric,  that  they  disappear  for  a  great  number  of  years.  If 
they  are  inhabited,  it  must  b^  by  a  species  of  beings 
very  different,  not  only  from  the  inhabitants  of  this,  but 
from  those  of  any  of  the  other  planets,  as  they  must  ex- 
perience the  greatest  vicissitudes  of  heat  and  cold  ;  one 
part  of  their  orbit  being  so  near  the  si^n,  that  their  heat, 
when  there,  is  computed  to  be  greater  than  that  of  red- 
hot  iron  ;  in  this  part  of  its  orbit,  the  comet  emits  a  lu- 
minous vapour,  called  the  tail,  which  it  gradually  loses 
as  it  recedes  from  the  sun  ;  and  the  comit  itself  totally 
disappears  from  our  sight,  in  the  more  distant  parts  of 
its  orbit,  which  extends  considerably  beyond  that  of  the 
furthest  planet. 

The  number  of  comets  belonging  to  our  system,  can- 
not  be  ascertained,  as  some  of  them  are  whole  centuries 
before  they  make  their  re-appearance.  The  numbers  that 
are  known  by  their  regular  re-appearance  is  only  three. 

Emily,     Pray,  Mrs,  B.  what  are  the  constellations  ? 

Mrs.  B.  They  are  the  fixed  stars,  which  the  an- 
cients, in  order  to  recognise  them,  lorrtied  into  ^rouj :es, 
and  gave  the  names  of  the  figures,  whicli  you  find  deli- 


0?%- 


^^.    Z'. 


I//       / 

/ 

/ 

\ 

\ 

\ 

)A- 

/             / 
c 

{ 

r 

^ 

J 

f\ 

\          \ 

K 

-^ 

1 

i 

1 

1 • _D 

/             / 

kk 

1 

im4. 

]^^<^^^'^ 

\ 

G^C^^^ 

'^:::::^^^^ 

Tul.  hv  J.Y.Huinpluvvs  Pha^t 


ON  THE  PLANETS.  129 

Boated  on  the  celestial  globe.  In  order  to  show  their 
proper  situations  in  the  heavens,  they  should  be  painted 
on  the  internal  surface  of  a  hollow  sphere,  from  the 
centre  of  which  you  should  view  them  ;  you  would  then 
behold  them,  as  they  appear  to  be  situated  in  the  hea- 
vens. The  twelve  constellations,  called  the  signs  of 
the  zodiac,  are  those  which  are  so  situated,  that  the  earth  . 
in  its  annual  revolution  passed  directly  between  them 
and  the  sun.  Their  names  are  Aries,  Taurus,  Gemini, 
Cancer,  Leo,  Virgo,  Libra,  Scorpio,  Sagittarius,  Capri- 
cornus,' Aquarius,  Pisces  ;  the  whole  occupying  a  com- 
plete circle,  or  broad  belt,  in  the  heavens,  called  the 
zodiac,  (plate  VIIL  fig.  L)  Hence,  a  right  line  drawn 
from  the  earthy  and  passing  through  the  sun,  would 
reach  one  of  these  constellations,  and  the  sun  is  said 
to  be  in  that  constellation  at  which  the  line  terminates : 
thus,  when  the  earth  is  at  A,  the  sun  would  appear  to  be 
in  the  constellation  or  sign  Aries  ;  when  the  earth  is  at 
B,  the  sun  would  appear  in  Cancer;  when  the  earth 
was  at  C,  the  sun  would  be  in  Libra;  and?  when  the 
earth  was  at  D,  the  sun  would  be  in  Capricorn.  This 
circle,  in  which  the  sun  thus  appears  to  move,  end 
which  passes  through  the  middle  of  the  zodiac,  is  called 
the  ecliptic. 

Caroline.  But  many  of  the  stars  in  these  constella- 
tions appear  beyond  the  zodiac. 

Mrs,  B,  We  have  no  means  of  ascertaining  the 
distance  of  the  fixed  stars.  When,  tlierefore,  they  are 
said  to  be  in  the  zodiac,  it  is  merely  implied,  that  they 
are  situated  in  that  direction,  and  that  tliey  shine  upon 
us  through  that  portion  of  the  heavens,  which  we  call 
the  zodiac. 


b 


130  ON  THE  PLANETS. 

Emily,  But  are  not  those  large  bright  stars,  which 
are  called  stars  of  the  first  magnitude,  nearer  to  us, 
than  those  small  ones  which  we  can  scarcely  discern  ? 

Mrs.  B,  It  may  be  so ;  or  the  difference  of  size  and 
brilliancy  of  the  stars  may  proceed  from,  their  difference 
of  dimensions;  this  is  a  point  which  astronomers  are 
not  enabled  to  determine.  Considering  them  as  suns, 
I  see  no  reason  why  difl^rent  suns  should  not  vary  in 
dimensions,  as  well  as  the  planets  belonging  to  them. 

Emily,  What  a  wonderful  and  beautiful  systeni  this 
is,  and  how  astonishing  to  think  that  every  fixed  star 
may  probably  be  attended  by  a  similar  train  of  planets  ! 

Caroline.  You  will  accuse  me  of  being  very  incre- 
dulous, but  I  cannot  help  still  entertaining  some  doubts, 
and  fearing  that  there  is  more  beauty  than  truth  in  this 
system.  It  certainly  may  be  so ;  but  there  does  not 
appear  to  me  to  be  sufficient  evidence  to  prove  it.  It 
seems  so  plain  and  obvious  that  the  earth  is  motionless, 
and  that  the  sun  and  stars  revolve  round  it ;-— your  so- 
lar system,^you  must  allow,  is  directly  in  opposition  to 
the  evidence  of  our  senses. 

Mrs.  B.  Our  senses  so  often  mislead  us,  that  we 
should  not  place  implicit  reliance  upon  them. 

Caroline.  On  what  then  can  we  rely,  for  do  we  not 
receive  all  our  ideas  through  the  medium  of  our  senses  ? 

jy[rs.  B.  It  is  true  that  they  are  our  primary  source 
of  knowledge ;  but  the  mind  has  the  power  of  reflect- 
ing, judging,  and  deciding  upon  the  ideas  received  by 
the  organs  of  sense.  This  faculty,  which  we  call  rea- 
son, has  frequently  proved  to  us,  that  our  senses  are 
liable  to  err.  If  jou  have  ever  sailed  on  the  water, 
with  a  very  steady  breeze,  you  must  have  seen  the 


ON  THE  PLANETS.  131 

houses,  trees,  and  every  object  move,  while  you  were 
sailing. 

Caroline.  I  remember  thinking  so,  when  I  was  very 
young ;  but  I  now  know  that  their  motion  is  only  ap- 
parent. It  is  true  that  my  reason,  in  this  case,  corrects 
the  error  of  my  sight. 

Mrs.  B.  It  teaches  you,  that  the  apparent  motion  of 
the  objects  on  shore,  proceeds  from  your  being  yourself 
moving,  and  that  you  are  not  sensible  of  your  own  mo- 
tion, because  you  meet  with  no  resistance.  It  is  only 
when  some  obstacle  impedes  our  motion,  that  we  are 
conscious  pf  moving;  and  if  you  were  to  close  your 
eyes  when  you  were  sailing  on  calm  water,  with  a  steady 
wind,  you  would  not  perceive  that  you  moved,  for  you 
could  not  feel  it,  and  you  could  see  it  only  by  observ- 
ing the  change  of  place  of  the  objects  on  shore.  So  it 
is  with  the  motion  of  the  earth  ;  every  thing  on  its  sur- 
faccj^and  the  air  that  surrounds  it,  accompanies  it  in  its 
revolution  ;  it  meets  with  no  resistance  :  therefore,  like 
the  crew  of  a  vessel  sailing  with  a  fair  wind,  in  a  calm 
sea,  we  are  insensible  of  our  motion. 

Caroline.  But  the  principal  reason  why  the  crew  of 
a  vessel  in  a  calm  sea  do  not  perceive  their  motion,  is, 
because  they  move  exceedingly  slowly  ;  while  the  earth, 
you  say,  revolves  with  great  velocity, 

Mrs.  B.  It  is  not  because  they  move  slowly,  but 
because  they  move  steadily,  and  meet  with  no  irregular 
resistances,  that  the  crew  of  a  vessel  do  not  perceive 
their  motion;  for  they  would  be  equally  insensible  to 
it,  with  the  strongest  wind,  provided  it  were  steady, 
that  they  sailed  with  it,  and  that  it  did  not  agitate  the 
water ;  but  this  last  condition,  you  know,  is  not  possi- 


iS2  ON  THE  PLANETS. 

ble,  for  the  wind  will  always  produce  waves  which  offer 
more  or  less  resistance  to  the  vessel,  and  then  the  mo- 
tion becomes  sensible,  because  it  is  unequal. 

Caroline.  But,  granting  this,  the  crew  of  a  vessel 
have  a  proof  of  their  motion,  though  insensible,  which 
the  inhabitants  of  the  earth  cannot  have, — tne  appa- 
rent motion  of  the  objects  on  shore. 

Mrs,  B,  Have  we  not  a  similar  proof  of  the  earth's 
motion,  in  the  apparent  motion  of  the  sun  and  stars. 
Imagine  the  earth  to  be  sailing  round  its  axis,  and 
successively  passing  by  every  star,  which,  like  the 
objects  on  land,  we  suppose  to  be  moving*  instead  of 
ourselves.  I  have  heard  it  observed  by  an  aerial  tra- 
veller in  a  balloon,  that  the  earth  appears  to  sink  be- 
neath the  balloon,  instead  of  the  balloon  rising  above 
the  earth. 

It  is  a  law  which  we  discover  throughout  nature  and 
worthy  of .  its  great  Author,  that  all  its  purposes  are 
accomplished  by  the  mo'^i  simple  means  ;  and  what 
reason  have  we  to  suppose  this  law  infringed,  in  order 
that  we  may  remain  at  rest*  while  the  sun  arid  stars 
move  round  us ;  their  regular  motions,  which  are  ex- 
plainjed  by  the  laws  of  attraction  on  the  first  supposi- 
tion, would  be  unintelligible  on  the  last,  and  the  order 
and  harmony  of  the  universe  "be  destroyed.  .Think  what 
an  immense  circuit  the  sun  and  stars  would  make  daily, 
were  their  apparent  motions  real.  We  know  many  of 
them  to  be  bodies  more  considerable  than  our  earth; 
for  our  eyes  vainly  endeavour,  to  persuade  us,  that  they 
are  little  brilliants  sparkling  in  the  heavens,  while  sci- 
ence teaches  us  that  they  are  immense  spheres,  whose 
apparent  dimensions  are  diminished  by  distance.  Why 


ON  THE  PLANETS.  133 

then  should  these  enormous  globes  daily  traverse  such 
a  prodigious  space,  merely  to  prevent  the  necessity  of 
our  earth's  revolving  on  its  axis  ? 

Caroline,  I  think  I  must  now  be  convinced.  But 
you  will,  I  hope,  allow  me  a  little  time  to  familiarise 
myself  to  an  idea  so  different  from  that  which  I  have 
been  accustomed  to  entertain.  And  pray,  at  what  rate 
do  we  move  ? 

Mrs,  B.  The  motion  produced  by  the  revolution  of 
the  earth  on  its  axis,  is  about  eleven  miles  a  minute,  to 
an  inhabitant  of  London. 

Emily,  But  does  not  every  part  of  the  earth  move 
with  the  same  velocity  ? 

Mrs,  B,  A  moment's  reflection  would  convince  you 
of  the  contrary:  a  person  at  the  equator  must  *iOve 
quicker  than  one  situated  near  the  poles,  since  they 
both  perform  a  revolution  in  24  hours. 

Emily,  True,  the  equator  is  farthest  from  the  axis 
of  motion.  But  in  the  earth's  revolution  round  the  sun, 
every  part  must  move  with  equal  velocity? 

Mrs,  B,     Yes,  about  a  thousand  miles  a  minute. 

Caroline,  How  astonishing  ! — and  that  it  should  be 
possible  for  us  to  be  insensible  of  such  a  rapid  motion. 
You  would  not  tell  me  this  sooner,  Mrs.  B.,  for  fear  of 
increasing  my  incredulity. 

Before  the  time  of  Newton,  was  not  the  earth  sup- 
posed to  be  in  the  centre  of  the  system,  and  the  sun, 
moon,  an^  stars  to  revolve  round  it  ? 

Mr^,  B,  This  was  the  system  of  Ptolemy  in  an- 
cient times ;  but  as  long  ago  as  the  beginning  of  the 
sixteenth  century  it  was  discarded,  and  the  solar  sys- 
tem, such  as  I  Tiave  shown  you,  was  established  by  the 

M 


134  ON  THE  PLANETS, 

celebrated  astronomer  Copernicus,  and  is  hence  call- 
ed  the  Copernican  system.  But  the  theorj^  of  gravita- 
tion, the  source  from  which  this  beautiful  and  harmo- 
nious arrangement  flows,  we  owe  to  the  powerful  genius 
of  Newton,  who  lived  at  a  much  later  period. 

Emily,  It  appears,  indeed,  far  less  difficult  to  trace 
by  observation  the  motion  of  the  planets,  than  to  divine 
by  what  power  they  are  impelled  and  guided.  I  won- 
der how  the  idea  of  gravitation  could  first  have  occur- 
red to  Sir  Isaac  Newton  ? 

Mrs.  B,  It  is  said  to  have  been  occasioned  by  a 
circumstance  from  which  one  should  little  have  ex- 
pected so  grand  a  theory  to  have  arisen. 

During  the  prevalence  of  the  plague  in  the  year  1665, 
Newton  retired  into  the  country  to  avoid  the  conta- 
gion :  when  sitting  one  day  in  his  orchard,  he  observed 
an  apple  fall  from  a  tree,  and  was  led  to  consider  what 
could  be  the  cause  which  brought  it  to  the  ground. 

Caroline,  If  I  dared  to  confess  it,  Mrs.  B.,  I  should 
say  that  such  an  enquiry  indicated  rather  a  deficiency 
than  a  superiority  of  intellect.  I  do  not  understand 
how  any  one  can  wonder  at  what  is  so  natural  and  so 
common. 

Mrs.  B.  It  is  the  mark  of  superior  genius  to  find 
matter  for  wonder,  observation,  and  research,  in  cir- 
cumstances which,  to  the  ordinary  mind,  appear  trivial, 
because  they  are  common,  and  with  which  they  are 
satisfied,  because  they  are  natural,  without  reflecting 
that  nature  is  our  grand  field  of  observation,  that  with- 
in it  is  contained  our  whole  store  of  knowledge ;  in  a 
word,  that  to  study  the  works  of  nature,  is  to  learn  t^ 
appreciate  and  admire  the  wisdom  of  God.  Thus^it  was 


ON  THE  PLANETS.  135 

the  simple  cireumstance  of  the  fall  of  an  apple,  which 
led  to  the  discovery  of  the  laws  upon  which  the  Coper- 
nican  system  is  founded  ;  and  whatever  credit  this 
system  had  obtained  before,  it  now  rests  upon  a  basis 
from  which  it  cannot  be  shaken. 

Emily,  This  was  a  most  fortunate  apple,  and  more 
worthy  to  be  commemorated  than  all  those  that  have 
been  sung  by  the  poets.  The  apple  of  discord  for 
which  the  goddesses  contended  ;  the  golden  apples  by 
which  Atalanta  won  the  race ;  nay,  even  the  apple 
which  William  Tell  shot  from  the  head  of  his  son,  can- 
not be  compared  to  this  ! 


eONVEKSATION  VIII. 


ON  THE  EARTH. 

OT    THE    TERRESTHIAL    globe. OF    THE  FIGURE  OF    THE    EARTH. 

OF    THE    PENDPLUM. OF    THE  VARIATION  OF    THE    SEASONS,    AND 

OF    THE    LENGTH    OF     DATS    AND    NIGHTS. OF     THE     CAUSES    OF 

THE    HEAT    OF    SUMMER. OF    SOLAR^     SIDERIAL,  AND    EaUAL    01> 

MEAN    TIME. 


Mrs.  B. 

As  the  earth  is  the  planet  in  which  we  are  the  most 
particularly  interested,  it  is  my  intention  this  morn- 
ing, to  explain  to  you  the  effects  resulting  from  its  an- 
nual and  diurnal  motions ;  but  for  this  purpose  it  will 
be  necessary  to  make  you  acquainted  with  the  terrestri- 
al globe:  you  have  not  either  of  you,  I  conclude,  leirnt 
the  use  of  the  globes  ? 

Caroline.  No ;  I  once  indeed  learnt  by  heart  the 
names  of  the  lines  marked  on  the  globe,  but  as  I  was 
informed  they  were  only  imaginary  divisions,  they  did 
not  appear  to  me  worthy  q/  much  attention,  and  were 
soon  forgo tton. 

M  2> 


138  ON  THE  EARTH. 

Mrs,  B,  You  suppose,  then,  that  astronomers  had 
been  at  the  trouble  of  inventing  a  number  of  lines 
to  little  purpose.  It  will  be  impossible  for  me  to  ex- 
plain to  you  the  particular  effects  of  the  earth's  motion 
without  your  having  acquired  a  knowledge  of  these 
lines  :  in  plate  VIII.  fig.  2.  you  will  find  them  all  deli- 
iieated ;  and  you  must  learn  them  perfectly  if  you  wish 
to  make  any  proficiency  in  astronomy. 

Caroline,  I  was  taught  them  at  so  early  an  age  that 
I  could  not  understand  their  meaning;  and  I  have  often 
heard  you  say  that  the  only  use  of  words  was  to  con- 
vey ideas. 

Mrs,  B.  The  names  of  these  lines  would  have  con- 
veyed ideas  of  the  figures  they  weie  designed  to  ex- 
press, though  the  use  of  these  figures  might  at  that  tin>e 
have  been  too  difficult  for  you  to  understand.  Child- 
hood is  the  season  when  impressions  on  the  memory  are 
most  strongly  and  most  easily  made :  it  is  the  period  at 
which  a  large  stock  of  ideas  should  be  treasured  up, 
the  application  of  which  we  may  learn  when  the  under- 
standing is  more  developed.  It  is,  I  think,  a  very  mis- 
taken notion  that  children  should  be  taught  such  things 
only,  as  they  can  perfectly  understand. '  Had  you  been 
early  made  acquainted  with  the  terms  which  relate  to 
figure  and  motion,  how  much  it  would  have  facilitated 
your  progress  in  natural  philosophy.  I  have  been  obli- 
ged to  confine  myself  to  the  most  common  and  famil/ar 
expressions,  in  explaining  the  laws  of  nature,  though  I 
am  convinced  that  appropriate  and  scientific  terms 
would  have  conveyed  more  precise  and  accurate  ideas  ; 
but  I  was  afraid  of  not  being  understood. 

Emily,      You   may  depend  upon  our  learning  the 
names  of  the^e  lines  thoroughly,  Mrs.  B. ;  but  before 


ON  THE  EARTH,  139 

we  commit  them  to  memory,  will  you  have  the  good- 
ness to  explain  them  to  us  ? 

Mrs,  B.  Most  willingly.  This  globe,  or  sphere, 
represents  the  earth;  the  line  which  passes  through 
its  centre,  and  on  which  it  turns,  is  called  its  axis, 
and  the  two  extremities  of  the  axis  A  and  B,  are  the 
poles,  distingrnshed  by  the  names  of  the  north  and 
south  pole.  The  circle  C  D,  which  divides  th§*globe 
into  two  equal  parts  between  the  poles,  is  called 
the  equator,  or  equinoctial  line  ;  that  part  of  the  globe 
to  the  north  of  the  equator  is  the  northern  hemisphere  ; 
that  part  to  the  south  of  the  equator,  the  southern  hem- 
isphere. The  small  circle  E  F,  which  surrounds  the 
north  pole,  is  called  the  arctic  circle  ;  that  G  H,  which 
surrounds  the  south  pole,  the  antarctic  circle.  There  are 
two  intermediate  circles  between,  the  polar  circles  and 
the  equator  ;  that  to  the  north,  1  K,  called  the  tropic  of 
Cancer;  that  to  the  south,  L  M,  called  the  tropic  of 
Capricorn.  Lastly,  this  circle,  L  K,  which  divides  the 
globe  into  two  equal  parts,  crossing  the  equator  and  ex- 
tending northward  as  far  as  the  tropic  of  Cancer,  and 
southward  as  far  as  the  tropic  of  Capricorn,  is  called 
the  ecliptic.  The  delineation  of  the  ecliptic  on  the  ter- 
re'^trial  globe  is  not  without  danger  of  conveying  false 
ideas  ;  for  the  ecliptic  (as  I  have  before  said)  is  an  ima- 
ginary circle  in  the  heavens  passing  through  the  middle 
of  the  zodiac,  and  situated  in  the  plane  of  the  earth's 
orbit. 

Caroline,     I  do  not  understanil  the  meaning  of  the 
plane  of  the  earth's  orbit. 

•jMrs,  B,     A   plane,  or  plain,  is  an  even  level  sur- 
face.   Let  us  suppose  a  bmaoth  thin  solid  plane  cut- 


140  ON  THE  EARTH. 

ting  the  sun  through  the  centre,  extending  out  as  far  as 
the  fixed  stars,  and  terminating  in  a  circle  which  passes 
through  the  middle  of  the  zodiac ;  in  this  plane  the 
earth  would  move  in  its  revolution  round  the  sun  ;  it  is 
therefore  called  the  plane  of  the  earth's  orbit,  and  the 
circle  in  which  this  plane  cuts  the  signs  of  the  zodiac 
is  tli^cliptic.  Let  the  fig.  1.  plate  IX.  represent  such 
a  plane,  S  the  sun,  E  the  earth  with  its  orbit,  and 
A  B  C  D  the  ecliptic  passing  through  the  middle  of  the 
zodiac. 

Emily.  If  the  ecliptic  relates  only  to  the  heavens, 
why  is  it  described  upon  the  terrestrial  globe  ^ 

Mrs,  B.  It  is  convenient  for  the  demonstration  of  a 
variety  of  problems  in  the  use  of  the  globes;  and  be- 
sides, the  obliquity  of  this  circle  to  the  equator  is  ren- 
dered more  conspicuous  by  its  being  described  on  the 
same  globe;  and  the  obliquity  of  the  ecliptic  shows  the 
inclination  of  the  earth's  axis  to  the  plane  of  its  orbit. 
But  to  return  to  fig.  2.  plate  VIII. 

The  spaces  between  the  several  parallel  circles  on 
the  terrestrial  globe  are  called  zones ;  that  which  is 
comprehended  between  the  tropics  is  distinguished  by 
the  name  of  the  torrid  zone  ;  the  spaces  which  extend 
from  the  Iropics.to  the  polar  circles,  the  north  and  south 
temperate  zones  ;  and  the  spaces  contained  within  the 
polar  circles,  the  frigid  zones. 

The  several  lines  which,  you  observe,  are  drawn  from 
one  pole  to  the  other,  cutting  the  equator  at  right  an- 
gles, are  called  meridians.  When  any  one  of  these 
meridians  is  exactly  opposite  the  sun  it  is  mid-day,  or 
twelve  o'clock  in  the  day,  with  all  the  places  situated 


Fi-^.  I- 


B>h.  hy  J.Y.Hujiiphi-^ys  PldJ.uif 


ON  THE  EARTH.  141 

en  that  meridian  ;  and,  with  the  places  situated  on  the 
opposite  meridian,  it  is  consequently  midnight. 

Emilij,  To  places  situated  equally  distant  from 
these  two  meridians,  it  must  then  be  six  o'clock  ? 

Mrs*  /?.  Yes ;  if  they  are  to  the  east  of  the  sun's 
meridian  it  is  six  o'clock  in  the  afternoon,  because  the 
sun  will  have  previously  passed  over  them ;  if  to  the 
west,  it  is  six  o'clock  in  the  morning,  and  the  sun  will 
lie  proceeding  towards  that  meridian. 

Those  circles  which  divide  the  globe  into  tw©  equal 
parts,  such  as  the  equator  and  the  ecliptic,  are  called 
greater  circles ;  to  distinguish  them  from  those  which 
divide  it  into  two  unequal  parts,  as  the  tropics  and  po- 
lar circles,  which  are  called  lesser  circles.  All  circles 
are  divided  into  S60  equal  parts,  called  degrees,  and  de-» 
grees  into  60  equal  parts,  called  minutes.  The  diameter 
of  a  circle  is  a  right  line  drawn  across  it,  and  passing 
through  the  centre ;  for  instance,  the  boundary  of  this 
sphere  is  a  circle,  and  its  axis  the  diameter  of  that  cir- 
cle ;  the  diameter  is  equal  to  a  little  less  than  one-third 
of  the  circumference.  Can  you  tell  me  nearly  how 
many  degrees  it  contains  ? 

Caroline.  It  must  b^  something  less  than  one-third 
of  360  degrees,  or  nearly  120  degrees. 

Mrs.  B.  Right ;  now  Emily  you  may  tell  me  exact- 
ly how  many  degrees  are  contained  in  a  meridian  ^ 

Emily.  A  meridian,  reaching  from  one  pole  to  the 
other,  is  half  a  circle,  and  must  therefore  contain  180 
degrees. 

Mrs.  B.  Very  well ;  and  what  number  of  degrees 
are  there  from  the  equator  to  the  poles  ? 

Caroline.     The  equator  being  equally  distant  from 


142  ON  THE  EARTH. 

either  pole,  that  distance  must  be  half  of  a  meridiaH, 
or  a  quarter  of  the  circumference  of  a  circle,  and  con- 
tain 90  degrees. 

Mrs,  B,  Besides  the  usual  division  of  circles  into 
degrees,  the  ecliptic  is  divided  into  twelve  equal  parts, 
called  signs,  which  bear  the  name  of  the  constellations 
through  which  this  circle  passes  in  the  heavens.  The 
degrees  measured  on  the  meridians  from  north  to  south, 
or  south  to  north,  are  called  degrees  of  latitude  ;  those 
measured  from  east  to  west  on  the  equator,  the  ecliptic, 
or  any  of  the  lesser  circles  are  called  the  degrees  of 
longitude  ;  hence  these  circles  bear  the  name  of  longi- 
tudinal circles;  they  are  also  called  parallels  of  latitude. 

Emily.  The  degrees  of  longitude  must  then  vary  in 
length  according  to  the  dimensions  of  the  circle  on 
which  they  are  reckoned ;  those,  for  instance,  at  the 
polar  circles  w^ill  be  considerably  smaller  than  those 
at  the  equator  ? 

Mrs,  B.  Certainly;  since  the  degrees  of  circles  of 
different  dimensions  do  not  vary  in  number,  they  must 
necessarily  vary  in  length.  The  degrees  of  latitude, 
you  may  observe,  never  vary  in  length ;  for  the  meri- 
dians on  which  they  are  reckoned  are  all  of  the  same 
dimensions. 

Emily.    And  of  what  length  is  a  degree  of  latitude  ? 

Mrs.  B.  Sixty  geographical  miles,  which  is  equal 
to  69 J  English  statute  miles. 

Emily.  The  degrees  of  longitude  at  the  equator 
must  then  be  of  the  same  dimensions  ? 

Mrs.  B.  They  would,  were  the  earth  a  perfect 
sphere;  but  its  form  is  not  exactly  spherical,  being 
somewhat  protuberant  about  the  equator,  and  flattened 


®ISr  THE  EARTH.  U3 

towards  the  poles.  This  form  is  supposed  to  proceed 
from  the  superior  action  of  the  centrifugal  power  at  the 
equator. 

Caroline,  I  thought  I  had  understood  the  centrifugal 
force  perfectly,  but  I  do  not  comprehend  its  efiect  in 
this  instance. 

Mrs.  B,  You  know  that  the  revolution  of  the  earth 
on  its  axis  must  give  every  particle  a  tendency  to  Hy 
off  from  the  centre,  that  this  tendency  is  stronger  or 
weaker  in  proportion  to  the  velocity  with  which  the 
particle  moves  ;  now  a  particle  situated  near  one  of  the 
polar  circles  makes  one  rotation  in  the  same  space  of 
time  as  a  particle  at  the  equator  ;  the  latter,  therefore, 
having  a  much  larger  circle  to  describe,  travels  pro- 
portionally faster,  consequently  the  centrifugal  force 
is  much  stronger  at  the  equator  than  at  the  polar  cir- 
cles :  it  gradually  decreases  as  you  leave  the  equator 
and  approach  the  poles,  where,  as  there  is  no  rotatory 
motion,  it  entirely  ceases.  Supposing,  therefore,  the 
earth  to  have  been  originally  in  a  fluid  state,  the  parti- 
cles in  the  torrid  zone  would  recede  much  farther  from 
the  centre  than  those  in  the  frigid  zones ;  thus  the  po- 
lar regions  would  become  flattened,  and  those  about  the 
equator  elevated. 

Caroline,  I  did  not  consider  that  the  particles  in  the 
neighbourhood  of  the  equator  move  with  greater  velo- 
city than  those  about  the  poles  ;  this  was  the  reason  I 
could  not  understand  you. 

Mrs.  B,  You  must  be  careful  to  remember,  that 
those  parts  of  a  body  which  are  farthest  from  the  cen- 
tre of  motion  must  move  with  the  greatest  velocity : 
the  axis  of  the  earth  is  the  centre  of  its  diurnal  motion. 


144  ON  THE  EARTH 

and  the  equatorial  regions  the  parts  raost  distant  from 
the  axis. 

Caroline,  My  head  then  moves  faster  than  my  feet ; 
and  upon  the  summit  of  a  mountain  we  are  carried 
round  quicker  than  in  a  valley  ? 

Mrs,  B,  Certainly,  your  head  is  more  distant  from 
the  centre  of  motion,  than  your  {'qqI  ;  the  mountain-top 
than  the  valley :  and  the  more  distant  any  part  of  a 
body  is  from  the  centre  of  motion,  the  larger  is  the  cir- 
cle it  will  describe,  and  the  greater  therefore  must  be 
its  velocity. 

Emily.  I  have  been  reflecting,  that  if  the  earth  is 
not  a  perfect  circle.... 

Mrs,  B,  A  sphere  you  mean,  my  dear  ;  a  circle  Is 
a  round  line,  every  part  of  which  is  equally  distant  from 
the  centre  ;  a  sphere  or  globe  is  a  round  body,  the  sur- 
face of  which  is  every  where  equally  distant  from  the 
centre. 

Emily^  If,  then,  the  earth  is  not  a  perfect  sphere, 
but  prominent  at  the  equator,  and  depressed  at  the  poles, 
would  not  a  body  weigh  heavier  at  the  equator  than  at 
the  poles  ?  For  the  earth  being  thicker  at  the  equator, 
the  attraction  of  gravity  perpendicularly  downwards 
must  be  stronger. 

Mrs,  B,  Your  reasoning  has  some  plausibility,  but 
I  am  sorry  to  be  obliged  to  add,  that  it  is  quite  erro- 
neous ;  for  the  nearer  any  part  of  the  surface  of  a  bo- 
dy is  ti)  the  centre  of  attraction,  the  more  strongly  it 
is  attracted  ;  because  the  most  considerable  quantity  of 
matter  is  about  that  centre.  In  regard  t*y  its  effects, 
you  might  consider  the  power  of  gravity,  as  that  of  a 
magnet  placed  at  the  centre  of  attraction. 


ONTHEEAHTH.  145 

Emily,  But  were  you  to  penetrate  deep  into  the 
earth,  would  gravity  increase  as  you  approached  the 
centre  ? 

Mrs.  B.  Certainly  not ;  I  am  referring  only  to  any 
situation  on  the  surface  of  the  earth.  Were  you  to  pen- 
etrate into  the  interior,  the  attraction  of  the  parts  above 
you  would  counteract  that  of  the  parts  beneath  you, 
and  consequently  diminish  the  power  of  gravity  in  pro- 
portion as  you  approached  the  centre ;  and  if  you  reached 
that  point,  being  equally  attracted  by  the  parts  all 
around  you,  gravity  would  cease,  and  you  would  be 
without  weight. 

Emily,  Bodies  then  should  weigh  less  at  the  equa- 
tor than  at  the  poles,  since  they  are  more  distant  from 
the  centre  of  gravity  in  the  former  than  in  the  latter 
situation. 

Mrs,  B.  And  this  is  really  the  case  ;  but  the  dif- 
ference of  weight  would  be  scarcely  sensible,  were  it 
not  augmented  by  another  circumstance. 

Caroline,  And  what  is  this  singular  circumstance 
which  seems  to  disturb  the  laws  of  nature  ? 

Mrs,  B.  One  that  you  are  well  acquainted  with,  as 
conducing  more  to  the  preservation  than  the  destruc- 
tion of  order, — the  centrifu2;al  force.  This  we  have 
just  observed  to  be  stronger  at  the  equator  ;  and  as  it 
tends  to  drive  bodies  from  the  centre,  it  is  necessarily 
opposed  to,  and  must  lessen  the  power  of  gravity 
which  attracts  them  towards  the  centre.  We  accor- 
dingly find  that  bodies  weigh  lightest  at  the  equator, 
where  the  centrifugal  force  is  greatest;  and  heaviest  at 
the  poles,  where  this  power  is  least. 


146  ON  THE  EARTH. 

Caroline,  Has  the  experiment  been  made  in  these 
different  situations  ? 

Mrs,  B,  Lewis  XIV.,  of  France,  sent  philosophers 
both  to  the  equator  and  to  Lapland  for  this  purpose  : 
the  severity  of  the  climate,  and  obstruction  of  the  ice, 
has  hitherto  rendered  every  attempt  to  reach  the  pole 
abortive ;  but  the  difference  of  gravity  at  the  equator 
and  in  Lapland  is  very  perceptible. 

Caroline,  Yet  I  do  not  comprehend,  how  the  diff^er- 
ence  of  weight  couid  be  ascertained  ;  for  if  the  body  un- 
der trial  decreased  in  weight,  the  weight  which  was  op- 
posed to  it  in  the  opposite  scale  must  have  diminished 
in  the  same  proportion.  For  instance,  if  a  pound  of  sugar 
did  not  weigh  so  heavy  at  the  equator  as  at  the  poles, 
the  leaden  pound  Which  served  to  weigh  it,  would  not 
be  so  heavy  either:  therefore  they  would  still  balance 
each  other,  and  the  different  force  of  gravity  could  not 
fee  ascertained  by  this  means. 

Mrs,  B,  Your  observation  is  perfectly  just :  the  dif- 
ference of  gravity  of  bodies  situated  at  the  poles  and  at 
the  equator  cannot  be  ascertained  by  weighing  them  ; 
a  pendulum  was  therefore  used  for  that  purpose. 

Caroline,  What,  the  pendulum  of  a  clock  ?  how' 
^uld  that  answer  that  purpose  ? 

Mrs,  B,  A  pendulum  consists  of  a  line,  or  rod,  to 
one  end  of  whi'^h  a  weight  is  attached,  and  it  is  sus- 
pended by  the  other  to  a  fixed  point,  about  which  it  is 
made  to  vibrate.  Without  being  put  in  motion,  a  pen- 
d  ;lum,  like  a  plumb  line,  hangs  perpendicular  to  the 
g^'Meral  surface  of  the  earth,  by  which  it  is  attracted ; 
but,  it"  you  raise  a  pendulum,  gravity  will  bring  it  back 
W  Its  perpendicular  position.    It  will,  however,  not  re- 


ON  THE  EARTH.  147 

main  stationary  there,  for  the  velocity  it  has  received 
during  its  descent  will  impel  it  onwards,  and  it  will 
rise  on  the  opposite  side  to  an  equal  height ;  from  thence 
it  is  broujrht  back  by  gravity,  and  again  driven  by  the 
impulse  of  its  velocity. 

Caroline.  If  so,  the  motion  of  a  pendulum  would  be 
perpetual,  and  I  thought  you  said,  that  there  was  no 
perpetual  motion  on  the  earth. 

Mrs,  B,  The  motion  of  a  pendulum  is  opposed  by 
the  resistance  of  the  air  in  which  it  vibrates,  and  by 
the  friction  of  the  part  by  which  it  is  suspended  :  were 
it  possible  to  remove  these  obstacles,  the  motion  of  a 
pendulum  would  be  perpetual,  and  its  vibrations  per- 
fectly regular:  being  of  equal  distances,  and  performed 
in  equal  times. 

Emily,  That  is  the  natural  result  of  the  uniformity 
of  the  power  which  produces  these  vibrations,  for  the 
force  of  gravity  being  always  the  same,  the  velocity  of 
the  pendulum  must  consequently  be  uniform. 

Caroline.  No,  Emily,  you  are  mistaken  ;  the  cause 
is  not  always  uniform,  and  therefore  the  effect  will  not 
be  so  either.  I  have  discovered  it,  Mrs.  B. ;  since  the 
force  of  gravity  is  less  at  the  equator  than  at  the  poles, 
the  vibrations  of  the  pendulum  will  be  slower  at  the 
equator  than  at  the  poles. 

Mrs,  B,  You  are  perfectly  right,  Caroline  ;  it  was 
by  this  means  that  the  difference  of  gravity  vi^as  dis- 
covered, and  the  true  figure  of  the  earth  ascertained. 

Emily,  But  how  do  they  contrive  to  regulate  their 
time  in  the  equatorial  and  polar  regions  ?  for,  since  in 
this  part  of  the  earth  the  pendulum  of  a  clock  vibrates 
exactly  OQce  in  a  second,  if  it  vibrates  faster  at  tKje 


i45  ON  THE  EARTH. 

poles  and  slower  at  the  equator,  the  inhabitants  must 
regulate  their  clocks  in  a  different  manner  from  ours. 

Mrs,  B,  The  only  alteration  required  is  to  lengthen 
the  pendulum  in  one  case,  and  to  shorten  it  in  the  other; 
for  the  velocity  of  the  vibrations  of  a  pendulum  de- 
pends on  its  length  ;  and  when  it  is  said,  that  a  pendu- 
lum vibrates  quicker  at  the  pole  than  at  the  equator,  it 
is  supposing  it  to  be  of  the  same  length.  A  pendulum 
which  vibrates  a  second  in  this  latitude  is  36j  inches 
long.  In  order  to  vibrate  at  the  equator  in  the  same 
space  of  time,  it  must  be  lengthened  by  the  addition  of 
a  few  lines ;  and  at  the  poles,  it  must  be  proportionally 
shortened. 

1  shall  now,  1  think,  be  able  to  explain  to  you  the 
variation  of  the  seasons,  and  the  difference  of  the  length 
of  the  days  and  nights  in  those  seasons ;  both  effects 
resulting  from  the  same  cause. 

In  moving  round  the  sun,  the  axis  of  the  earth  is  not 
perpendicular  to  the  plane  of  its  orbit.  Supposing  this 
round  table  to  represent  the  plane  of  the  earth's  orbit, 
and  this  little  globe,  which  has  a  wire  passing  through 
it,  representing  the  axis  and  poles,  we  shall  call  the 
earth  ;  in  moving  round  the  table,  the  wire  is  not  per- 
pendicular to  it,  but  oblique. 

Emily,  Yes,  1  understand  the  earth  does  not  go 
round  the  sun  in  an  upright  position,  its  axis  is  slanting 
or  oblique. 

Mrs,  B,  All  the  lines,  which  you  learnt  in  your  last 
lesson,  are  delineated  on  this  little  globe ;  you  must 
consider  the  ecliptic  as  repr^enting  the  plane  of  the 
earth's  orbit ;  and  the  equator,  which  crosses  the  eclip- 
tic in  two  places,  shows  the  degree  of  obliquity  of  the 


ON  THE  EARTH.  149 

axis  of  the  earth  in  that  orbit,  which  is  exactly  23§  de- 
grees. The  points  in  which  the  ecliptic  intersects  the 
equator  are  called  nodes. 

But  I  believe  I  shall  make  this  clear  to  you  by  revolv- 
ing the  little  globe  round  a  candle,  which  shall  repre- 
sent the  sun.     (Plate  IX.  fig,  2.) 

As  I  now  held  it,  at  A,  you  see  it  in  the  situation  in 
which  it  is  in  the  midst  of  summer,  or  what  is  called 
the  summer  solstice,  which  is  on  the  21st  of  June. 

Emily.  You  hold  the  wire  awry,  I  suppose,  in  order 
to  show  that  the  axis  of  the  earth  is  not  upright? 

Mrs,  B,  Yes  ;  in  summer,  the  north  pole  is  inclin- 
ed towards  the  sun.  In  this  season,  therefore,  the  nor- 
thern hemisphere  enjoys  much  more  of  liis  rays  than  the 
southern.  The  sun,  you  see,  now  shines  over  the  whole 
of  the  north  frigid  zone,  and  notwithstanding  the  earth's 
diurnal  revolution,  which  I  imitate  by  twirling  the  ball 
on  the  wire,  it  will  continue  to  shine  upon  it  as  long  as 
it  remains  in  this  situation,  whilst  the  south  frigid  zone 
is  at  the  same  time  completely  in  obscurity. 

Caroline,  That  is  very  strange  :  1  never  before 
heard  that  there  ^as  constant  day  or  night  in  any  part 
of  the  world  !  How  much  happier  the  inhabitants  of 
the  north  frigid  y.one  must  be  than  those  of  the  southern  ; 
the  first  enjoy  unintf^rrupted  day,  while  the  last  are  in- 
volved in  perpetual  darkness. 

Mrs,  B,  You  judge  with  too  much  precipitation; 
examine  a  little  further,  and  you  will  find,  that  the  two 
frigid  zones  share  an  equal  fate. 

We  shall  now  make  the  earth  set  off  from  its  posi- 
tion in  the  summer  solstice,  and  carry  it  round  the  sun ; 
observe  that  the  pole  is  always  inclined  in  the  same  di^ 
n2 


150  ON  THE  EARTH. 

rection,  and  points  to  the  same  spot  in  the  heavens. 
There  is  a  fixed  star  situated  near  that  spot,  which  is 
hence  called  the  North  Polar  star.  Now  let  us  stop  the 
earth  at  B,  and  examine  it  in  its  present  situation ;  it 
has  gone  through  one  quarter  of  its  orbit,  and  is  arrived 
at  that  point  at  which  the  ecliptic  cuts  or  crosses  the 
equator,  and  which  is  called  the  autumnal  equinox. 

Emily.     That  is  then  one  of  the  nodes. 

The  sun  now  shines  from  one  pole  to  the  other,  just 
as  it  would  constantly  do,  if  the  axis  of  the  earth  were 
perpendicular  to  its  orbit. 

Mrs,  B,  Because  the  inclination  of  the  axis  is  now 
neither  towards  the  sun  nor  in  the  contrary  direction  ; 
at  this  period  of  the  year,  therefore,  the  days  and  nights 
are  equal  in  every  part  of  the  earth.  But  the  next 
step  she  takes  in  her  orbit,  you  see,  involves  the  north 
pole  in  darkness,  whilst  it  illumines  that  of  the  south  ; 
this  change  was  gradually  preparing  as  I  moved  the 
earth  from  summer  to  autumn  ;  the  arctic  circle,  which 
was  at  first  entirely  illumined,  began  to  have  short  nights, 
which  increased  as  the  earth  approached  the  autumnal 
equinox  ;  and  the  instant  it  passecf  that  point,  the  long 
night  of  the  north  pole  commences,  and  the  south  pole  be- 
gins to  enjoy  the  light  of  the  sun.  We  shall  now  make 
the  earth  proceed  in  its  orbit,  and  you  may  observe  that 
as  it  advances,  the  days  shorten,  and  the  nights  length- 
en, throughout  the  northern  hemisphere,  until  it  arrives 
at  the  winter  solstice,  on  the  21st  of  December,  when 
the  north  frigid  zone  is  entirely  in  darkness,  and  the 
southern  has  uninterrupted  day-light. 

Caroline.  Then  after  all,  the  sun  which  I  thought 
so  partial,  confers  his  favours  equally  on  all. 


ON  THE  EARTH.  151 

Mrs.  B.  Not  so  neither  :  the  inhabitants  of  the  tor- 
lid  zone  have  much  more  heat  than  we  have, as  the  sun's 
rays  fall  perpendicularly  on  them,  while  they  shine  ob- 
liquely on  the  rest  of  the  world,  and  almost  horizontally 
on  the  poles  ;  for  during  their  Ions;  day  of  six  months, 
the  sun  moves  round  their  horizon  without  either  rising 
or  setting ;  the  onlv  observable  difference,  is  that  it  is 
more  elevated  by  a  few  degrees  at  mid-day,  than  at  mid- 
night. 

Emily.  To  a  person  placed  in  the  temperate  zone, 
in  the  situation  in  which  we  are  in  England,  the  sun 
will  shine  neither  so  obliquely  as  it  does  on  the  poles, 
nor  so  vertically  as  at  the  equator ;  but  its  rays  will 
fall  upon  him  more  obliquely  in  autumn  and  winter, 
than  in  summer. 

Caroline.  And  therefore,  the  inhabitants  of  the  tem- 
perate zones,  will  not  have  merely  one  day  and  one 
night  in  the  year  as  happens  at  the  poles,  nor  will  they 
have  equal  days  and  equal  nights  as  at  the  equator ;  but 
their  days  and  nights  will  vary  in  length,  at  different 
times  of  the  year,  according  as  their  respective  poles 
incline  towards  or  from  the  sun,  and  the  difference  will 
be  greater  in  proportion  to  their  distance  from  the 
«c|uator. 

Mrs.  B.  We  shall  now  follow  the  earth  through  the 
other  half  of  her  orbit,  and  you  will  observe,  that  now 
exactly  the  same  effect  takes  place  in  the  southern  hem- 
isphere, as  what  we  have  just  remarked  in  the  northern. 
Day  commences  at  the  south  pole  when  night  sets  in  at 
the  north  pole  ;  and  in  ever}^  other  part  of  the  southern 
hemisphere  the  days  are  longer  than  the  nights,  while, 
on  the  contrary,  our  nights  are  longer  than  our  days. 


152  ON  THE  EARTII. 

When  the  earth  arrives  at  the  vernal  equinox,  D,  where 
the  ecliptic  again  cuts  the  equator,  on  the  25th  of 
March,  she  is  situated,  with  respect  to  the  sun,  exactly 
in  the  same  position,  as  in  the  autumnal  equinox  ;  and 
the  (»nlj  difference  with  respect  to  the  earth,  is,  that  it 
is  now  autumn  in  the  southern  hemisphere,  whilst  it  is 
spring  with  us. 

Caroline.  Then  the  days  and  nights  are  again  every 
where  equal  ? 

Mrs.  B.  Yes,  for  the  half  of  the  globe  which  is  en- 
lightened, extends  exactly  from  one  pole  to  the  other, 
the  day  breaks  to  the  north  pole,  and  the  sun  sets  to  the 
south  pole ;  but  in  every  other  part  of  the  globe,  the 
day  and  night  is  of  twelve  hours  length,  hence  the 
word  equinoXj  Vt'hich  is  derived  from  the  Latin,  meaning 
equal  night. 

As  the  earth  proceeds  towards  summer,  the  days 
lengthen  in  the  northern  hemisphere,  and  shorten  in  the 
southern,  till  the  earth  reaches  the  summer  solstice, 
when  the  north  frigid  zone  is  entirely  illumined,  and 
the  southern  is  in  complete  darkness  ;  and  we  have  now 
brought  the  earth  again  to  the  spot  from  whence  we  first 
accompanied  her, 

SEmily.     This  is  indeed,  a  most  satisfactory  explana- 
tion of  the  seasons ;  and  the  more  [  h^arn,  the  more  I . 
admire  the  simplicity  of  means  by  which  such  wonder- 
ful effects  are  produced. 

Mrs.  B.  I  know  not  which  is  most  worthy  of  our 
admiration,  the  cause,  or  the  effect  of  the  earth's  revo- 
lution, round  the  sun.  The  mind  can  find  no  object  of 
contemplation,  more  sublime,  than  the  course  of  this 
magnificent  globe,  impelled  by'  the  combined  powers  jp 


ON  THE  EARTH.  156 

projection  and  attraction  to  roll  in  one  invariable  course 
around  the  source  of  lij^ht  and  heat :  and  what  can  be 
more  delightful  than  the  beneficent  effects  of  this  vivi- 
fying power  on  its  attendant  planet.  It  is  at  once  the 
grand  principle  which  animates  and  fecundates  nature, 

Emily.  There  is  one  circumstance  in  which  this  lit- 
tle ivory  globe  appears  to  me  to  diflfer  from  the  earth; 
it  is  not  quite  dark  on  that  side  of  it,  which  is  turned 
fiom  the  candle,  as  is  the  case  with  the  earth  when  nei- 
ther moon  nor  stars  are  visible. 

Jlrs.  B.  This  is  owin£^  to  the  light  of  the  candle  be- 
ing reflected  by  the  walls  of  the  room  on  every  part  of 
the  globe,  consequently  that  side  of  the  globe  on  which 
the  candle  does  not  directly  shine,  is  not  in  total  dark- 
ness. Now  the  skies  have  no  walls  to  reflect  the  sun's 
light  on  that  side  of  our  earth  which  is  in  darkness. 

Caroline,  I  beg  your  pardon,  Mrs.  B.,  I  think  that 
the  moon  and  stars  answer  the  purpose  of  walls  in  re- 
flecting the  sun's  light  to  us  in  the  night. 

Mrs.  B.  Very  well,  Caroline ;  that  is  to  say,  the 
moon  and  planets ;  for  the  fixed  stars,  you  know  shine 
by  their  own  light. 

Emily.  You  say,  that  the  superior  heat  of  the  equa- 
torial parts  of  the  earth,  arises  from  the  rays  falling 
perpendicularly  on  those  regions,  whilst  they  fall  ob- 
liquely on  these  more  northern  regions  ;  now  I  do  not 
understand  why  perpendicular  rays  should  afford  more 
heat  than  oblique  rays. 

•Mrs.  B.  You  need  only  hold  your  hand  perpendi- 
cularly over  the  candle,  and  then  hold  it  sideways  ob- 
liquely, to  be  sensible  of  the  difference. 

Emily.  I  do  not  doubt  the  fact,  but  I  wish  to  have 
it  explained. 


154  ON  THE  EARTH, 

Mrs.  B.  You  are  quite  right ;  if  Caroline  tiad  not 
been  satisfied  with  ascertaining  the  fact,  without  un- 
derstanding it,  she  would  not  have  brought  forward  the 
candle  as  an  illustration;  the  reason  why  you  feel  so 
much, more  heat  if  you  hold  your  hand  perpendicularly 
over  the  candle,  than  if  you  hold  it  sideways,  is  because 
a  stream  of  heated  vapour  constantly  ascends  from  the 
candle,  or  any  other  burning  body,  which  being  lighter 
than  the  air  of  the  room,  does  not  spread  laterally  but 
rises  perpendiculajrly,  and  this  led  you  to  suppose  that 
the  rays  were  hotter  in  the  Utter  direction.  Had  you 
reflected,  you  would  have  discovered  that  rays  issuing 
from  the  candle  sideways,  are  no  less  perpendicular  to 
your  hand  when  held  opposite  to  them,  than  the  rays 
which  ascend  when  your  hand  is  held  over  them. 

The  reason  why  the  sun's  rays  aftbrd  less  heat  when 
in  an  oblique  direction  than  when  perpendicular,  is  be- 
cause fewer  of  them  fall  upon  an  equal  portion  of  the 
earth;  this  will  be  understood  better  by  referring  to 
plate  X.  fig.  1,  which  represents  two  equal  portions  of 
the  sun's  rays,  shining  upon  different  parts  of  the  earth. 
Here  it  is  evident  that  the  same  quantity  of  rays,  fall 
on  the  space  A  B,  as  fall  on  the  space  B  C  ;  and  as 
A  E  is  less  than  B  C,  the  heat  and  light  will  be  muclr 
stronger  in  the  former  than  in  the  latter;  A  B,  you 
see  represents  the  equatorial  regions,  where  the  suiji 
shines  perpendicularly;  and  B  C,  the  temperate  and 
frozen  climates,  v^here  his  rays  fall  more  obliquely. 

Emilu,  This  accounts  not  only  for  the  greater  heat 
of  the  equatorial  regions,  but  for  the  greater  heat  of 
summer;  as  the  sun  'shines  less  obliquely  in  summer 
than  in  winter. 


fixZ/.  hy  JY.HntrLfjki'eys  FIixUlJ:; 


ON  THE  EARTH.  155 

Mrfi.  B.  This  you  will  see  exemplified  in  figure  2, 
in  which  the  earth  is  represented,  as  it  is  situated  on 
the  21^t  June,  and  England  receives  less  oblique  and 
consequently  a  greater  number  of  rays,  than  at  any 
other  season  ;  and  figure  3,  shows  the  situation  of  Eng- 
land on  the  2 1st  December,  when  the  rays  of  the  sun 
fall  most  obliquely  upon  her.  But  there  is  also  another 
reason  why  obllciue  rays  give  less  heat,  than  perpetidi- 
cular  rays;  which  is,  that  they  have  a  greater  portion 
of  the  atmosphere  to  traverse;  and  though  it  is  true, 
that  the  atmosphere*  is  itself  a  transparent  body,  freely 
admitting  the  passage  of  the  sun's  rays,  yet  it  is  always 
loaded  more  or  less  with  dense  and  foggy  vapour,  which 
the  rays  of  the  sun  cannot  easily  penetrate ;  therefore 
the  greater  the  quantity  of  atmosphere  the  sun's  rays 
have  to  pass  throu«;h  in  their  way  to  the  earth,  the  less 
heat  the}'  will  retain  when  they  reach  it  This  will  be 
better  un-derstood,  by  referring  to  figure  4.  The  dotted 
line  round  the  earth,  describes  the  extent  of  the  atmos- 
phere, and  the  lines  which  proceed  from  the  sun  to  the 
earth,  the  passage  of  two  equal  portions  of  the  sun*« 
rays  to  the  equatorial  and  polar  regions  ;  the  latter  you 
see,  from  its  greater  obliquity  passes  through  a  greater 
extent  of  atmosphere. 

Caroliup,  And  this,  no  doubt,  is  the  reason  why  th© 
sun  in  the  morninii;  and  the  evening  gives  so  much  les« 
heat,  than  at  mid-day. 

Mrs,  n.  The  diminution  of  heat,  morning  and  eve- 
nina:  is  certainly  owing  to  the  grp^iter  obliquity  of  the 
sun's  rays;  and  as  such  they  are  affected  by  both  the 
causes,  which  F  have  just  explained  to  you;  the  difficul- 
ty of  passing  tlirough  a  foggy  atmoS|^*here  is   perhaps 


156  «N  THE  EARTH. 

more  particularly  applicable  to  them,  as  mists  and  va- 
pours are  very  prevalent  about  the  time  of  sunrise  and 
sunset.  But  the  diminished  obliquity  of  the  sun's  i%ys, 
is  not  the  sole  cause  of  the  heat  of  summer ;  the  len^tK 
of  the  days  greatly  conduces  to  it;  for  the  fonger  the 
sun  is  above  the  horizon,  the  more  heat  he  will  commu- 
■icate  to  the  earth. 

Caroline.  Both  the  longest  days,  and  the  most  per- 
pendicular rays,  are  on  the  21st  of  June  ;  and  yet  the 
greatest  heat  prevails  in  July  and  August. 

Mrs,  B.  Those  parts  of  the  earth  which  are  once 
heated,  retain  the  heat  for  some  length  of  time,  and 
the  additional  heat  they  receive,  occasions  an  elevation 
of  temperature,  although  the  days  begin  to  shorten, 
and  the  sun's  rays  fall  more  obliquely.  For  the  same 
reason,  we  have  generally  more  heat  at  three  o'clock  in 
the  afternoon,  than  at  twelve  when  the  sun  is  on  the  me- 
ridian. 

Emily,  And  pray,  have  the  other  planets  the  same 
ricissitudes  of  seasons,  as  the  earth  ? 

Mrs,  B,  Some  of  them  more,  some  less,  according 
as  their  axes  deviate  more  or  less  from  the  perpendicu- 
lar to  the  plane  of  their  orbits.  The  axis  of  Jupiter  is 
nearly  perpendicular  to  the  plane  of  his  orbit ;  the 
axes  of  Mars  and  of  Saturn  are  each  inclined  at  angles 
of  about  sixty  degrees  ;  whilst  the  axis  of  Venus  is  be- 
lieved to  be  elevated  only  fifteen  or  twenty  degrees 
above  her  orbit ;  the  vicissitudes  of  her  seasons  must 
therefore  be  considerably  greater  than  ours.  F(»r  fur- 
ther particulars  respecting  the  planets,  1  shall  refer  joa 
to  Bunny  castle's  Introduction  to  Astronomy. 


ON  THE  EARTH.  157 

I  have  but  one  more  observation  to  make  to  you  rela- 
tive to  the  earth's  motion,  which  is,  that  although  we 
have  but  365  days  and  nights  in  the  year,  she  performs 
366  complete  revolutions  on  her  axis  during  that  time. 
Caroline,  How  is  that  possible  ?  for  every  complete 
revolution  must  bring  the  same  place  back  to  the  sun. 
It  is  now  just  twelve  o'clock,  the  sun  is,  therefore,  on 
our  meridian ;  in  twenty-four  hours  will  it  not  be  re- 
turned to  our  meridian  again,  and  will  not  the  earth 
have  made  a  complete  rotation  on  its  axis. 

Mrs.  B.  If  the  earth  had  no  progressive  motion  in 
its  orbit  whilst  it  revolves  on  its  axis,  this  would  be  the 
case ;  but  as  it  advances  almost  a  degree  westward  in 
its  orbit,  in  the  same  time  that  it  completes  a  revolution 
eastward  on  its  axis,  it  must  revolve  nearly  one  degree 
more  in  order  to  bring  the  same  meridian  back  to  the 
sun. 

Caroline.     Oh,  yes  !  it  will  require  as  much  more  of 

a  second  revolution  to  bring  the^ame  meridian  back  to 

the  sun,  as  is  equal  to  the  space  the  earth  has  advanced 

^     in  her  orbit,  that  is,  nearly  a  degree;  this  difference  is 

however,  very  little. 

Mrs,  B.  These  small  daily  portions  of  rotation  are 
each  equal  to  the  three  hundred  and  sixty-fifth  part  of 
a  circle,  which  at  the  end  of  the  year  amounts  to  one 
complete  rotation. 
*  J  Emily.  That  is  extremely  curious.  If  the  earth, 
then,  had  no  other  than  its  diurnal  motion,  we  should 
have  366  days  in  the  year. 

Mrs.  B.     We  should  have  366  days  in  the  same  pe- 
riod of  time  that  we  now  have  ^Qo ;  but  if  we  did  not 
o 


158  ON  THE  EARTH. 

revolve  round  the  sun,  we  should  have  no  natural  mean* 
of  computing  years. 

You  will  be  surprized  to  hear,  that  if  time  is  calcu- 
lated by  the  stars  instead  of  the  sun,  the  irregularity 
which  we  have  just  noticed  does  not  occur,  and  that  one 
complete  rotation  of  the  earth  on  its  axis,  brings  the 
same  meridian  back  to  any  fixed  star. 

Emily.  That  seems  quite  unaccountable  ;  for  the 
earth  advances  in  her  orbit  with  regard  to  the  fixed 
stars,  the  same  as  with  regard  to  the  sun. 

Mrs.  B,  True,  but  then  the  distance  of  the  fixed 
stars  is  so  immense,  that  our  solar  system  is  in  compa- 
rison to  it  but  a  spot,  and  the  whole  extent  of  the  earth's 
orbit  but  a  point  i  therefore,  whether  the  earth  remained 
stationary,  or  whether  it  revolved  in  its  orbit  during 
its  rotation  on  its  axis,  no  sensible  difference  would  be 
produced  with  regard  to  the  fixed  stars.  One  complete 
revolution  brings  the  same  meridian  back  to  the  same 
fixed  star ;  hence  the  fixed  stars  appear  to  go  round  the 
earth  in  a  shortervtime  than  the  sun  by  three  minutes  fif- 
ty-six seconds  of  time. 

Caroline.  These  three  minutes  fifty-six  seconds  is 
the  time  which  the  earth  takes  to  perform  the  addition- 
al three  hundred  and  sixty-fifth  part  of  the  circle,  in  or- 
der to  bring  the  same  meridian  back  to  the  sun. 

Mrs.  B.  Precisely.  Hence  the  stars  gain  every  day 
three  minutes  fifty-six  seconds  on  the  sun,  which  makes 
them  rise  that  portion  of  time  earlier  every  day. 

When  time  is  calculated  by  the  stars  it  is  called  si- 
dereal time,  when  by  the  sun  solar  or  apparent  time. 

Caroline.  Then  a  sidereal  day  is  three  minutes  fifty- 
aix  seconds  shorter  than  a  solar  day  of  twenty-four  hours. 


Plate  :xi. 


3ib.  hyJ.yjhwtphr^y^J'h£L,M'^ 


ON  THE  EARTH.  15^ 

Mrs.  B,  I  must  also  explain  to  you  what  is  meant 
hy  a  sidereal  yeai% 

The  common  year,  called  the  solar  or  tropical  year, 
containing  365  days  five  hours,  forty-eight  minutes^ 
and  fifty-two  seconds,  is  measured  from  the  time  the 
sun  sets  out  from  one  of  the  equinoxes,  or  solstices, 
till  it  returns  to  the  same  again  ;  but  this  year  is  com- 
pleted before  the  earth  has  finished  one  entire  revolution 
in  its  orbit. 

Emily.  I  thought  that  the  earth  performed  one  com- 
plete revolution  in  its  orbit  every  year ;  what  is  the 
reason  of  this  variation  ? 

Mrs.  B.  It  is  owing  to  the  spheroidal  figure  of  the 
earth.  The  elevation  about  the  equator  produces  much 
the  same  effect  as  if  a  similar  mass  of  matter,  collec- 
ted in  the  form  of  a  moon,  revolved  round  the  equator. 
When  this  moon  acted  on  the  earth  in  conjunction  with 
or  in  opposition  to  the  sun,  variations  in  the  earth's 
motion  would  be  occasioned,  and  these  variations  pro- 
duce what  is  called  the  precession  of  the  equinoxes. 

Emily.  Wliat  does  that  mean?  I  thought  the  equi- 
noctial points,  or  nodes,  were  fixed  points  in  the  hea- 
vens, in  which  the  equator  cuts  the  ecliptic. 

Mrs.  B.  These  points  are  not  quite  fixed,  but  have 
an  apparently  retrograde  motion,  that  is  to  say,  instead 
of  being  every  revolution  in  the  same  place,  they  move 
backwards.  Thus  if  the  vernal  equinox  is  at  A,  (fig.  1. 
plate  XL)  the  autumnal  one  will  be  at  B  instead  of  C, 
and  the  following  vernal  equinox  at  D  instead  of  at  A, 
as  would  be  the  case  if  the  equinoxes  were  stationary 
at  opposite  points  of  the  earth's  orbit. 


160  ON  THE  EARTH. 

Caroline,  So  that  when  the  earth  moves  from  one 
equinox  to  the  other,  though  it  takes  half  a  year  to  per- 
form the  journej,  it  has  not  travelled  through  half  its 
orbit. 

Mrs,  B,  And,  consequently,  when  it  returns  again 
to  the  first  equinox,  it  has  not  completed  the  whole  of 
its  orbit.  In  order  to  ascertain  when  the  earth  has  per- 
formed an  entire  revolution  in  its  orbit,  we  must  ob- 
serve when  the  sun  returns  in  conjunction  with  any 
fixed  star  ;  and  this  is  called  a  sidereal  year.  Supposing 
a  fixed  star  situated  at  E,  (fig.  1.  plate  XI.)  the  sun 
would  not  appear  in  conjunction  with  it  till  the  earth 
had  returned  to  A,  when  it  would  have  completed  its 

Emily.  And  how  much  longer  is  the  sidereal  than 
the  solar  year  ? 

Mrs,  B,  Only  twenty  minutes ;  so  that  the  variation 
of  the  equinoctial  points  is  very  inconsiderable.  I  have 
given  theni  a  greater  extent  in  the  figure  in  order  to  ren- 
der them  sensible. 

In  regard  to  time,  I  must  further  add,  that  the  earth's 
diurnal  motion  on  an  inclined  axis,  together  with  its 
annual  revolution  in  ah  elliptic  orbit,  occasions  so  much 
complication  in  its  motion,  as  to  produce  many  irregu- 
larities ;  therefore  true  equal  time  cannot  be  measured 
by  the  sun.  A  clock,  which  was  always  perfectly  cor- 
rect, would  in  some  parts  of  the  year  be  before  the  sun, 
and  in  other  parts  after  it.  There  arc  but  four  periods 
in  which  the  sun  and  a  perfect  clock  would  agree, 
which  is  the  1 5th  of  April,  the  l6th  of  June,  the  23d 
of  August,  and  the  24th  of  December. 


ON  THE  EARTH.  Idl 

Emily,  And  is  there  any  considerable  difference 
between  solar  time  and  true  time  ? 

Mrs,  B,  The  greatest  difference  amounts  to  between 
fifteen  and  sixteen  minutes.  Tables  of  equation  are 
constructed  for  the  purpose  of  pointing  out  and  correct- 
ing these  differences  between  solar  time  and  equal  or 
mean  time,  which  is  the  denomination  given  by  astro- 
nomers to  true  time. 


CONVERSATION  IX. 


ON  THE  MOON. 

OF   THE    moon's  MOTIOK. PHASES   OP   THE   MOOIT. — .ECLIPSES     OF 

THE    MOON. ECLIPSES    OF    JUPTTER's    MOONS. OF    THE    LATI- 
TUDE   AND    LONGITUDE. OF   THE    TRANSITS   OF    THE    INFERIOR 

PLANETS. — OF    THE   TIDES. 


Mrs.  B. 

We  shall  to-tlay  confine  our  attention  to  the  moon, 
which  offers  many  interesting  phenomena. 

The  moon  revolves  round  the  earth  in  the  space  of 
about  twenty-nine  days  and  a  half,  in  an  orbit  nearly 
parallel  to  that  of  the  earth,  and  accompanies  us  in 
our  revolution  round  the  sun. 

Emily,  Her  motion  then,  must  be  rather  of  a  com- 
plicated nature ;  for  as  the  earth  is  not  stationary,  but 
advances  in  her  orbit  whilst  the  moon  goes  round  her, 
the  moon  must  proceed  in  a  sort  of  progressive  circle. 

Mrs,  B,  That  is  true  ;  and  there  are  also  other  cir- 
cumstances which  interfere  with  the  simplicity  and  re- 


164  ON  THE  MOON. 

gularity  of  the  moon's  motion,  but  which  are  too  intri- 
cate for  you  to  understand  at  present. 

The  moon  always  presents  the  same  face  to  us,  by 
which  it  is  evident  that  she  turns  but  once  upon  her 
axis,  while  she  performs  a  revolution  round  the  earth; 
so  that  the  inhabitants  of  the  moon  have  but  one  day 
and  one  night  in  the  course  of  a  lunar  month. 

Caroline,  We  afford  them  however,  the  advantage 
of  a  magnificent  moon  to  enlighten  their  long  nights. 

Mrs,  B,  That  advantage  is  but  partial ;  for  since  we 
always  see  the  same  hemisphere  of  the  moon,  the  in- 
habitants of  that  hemisphere  alone  can  perceive  us. 

Caroline,  One  half  of  the  moon  then  enjoys  our 
light  every  night,  while  the  other  half  has  constantly 
nights  of  darkness.  If  there  are  any  astronomers  in 
those  regions,  they  would  doubtless  be  tempted  to  visit 
the  other  hemisphere,  in  order  to  behold  so  grand  a 
luminary  as  we  must  appear  to  them.  But,  pray,  do 
they  see  the  earth  under  all  the  changes  which  the 
moon  exhibits  to  us  ? 

,Mrs,  B,  Exactly  so.  These  changes  are  called  the 
phases  of  the  moon,  and  require  some  explanation.  In 
figure  2,  plate  XL  let  us  say  that  S  represents  the  sun, 
E  the  Earth,  and  A  B  C  D  the  moon  in  different  parts 
of  her  orbit.  Vvhen  the  moon  is  at  A,  her  dark  side 
being  turned  towards  the  earth,  we  shall  not  see  her  as 
at  a  ;  but  her  disappearance  is  of  very  short  duration, 
and  as  she  advances  in  her  orbit  we  perceive  her  under 
the  form  of  a  new  moon  ;  when  she  has  gone  through 
one-ei2;hth  of  her  orbit  at  B,  one  quarter  of  her  enlight- 
ened hemisphere  will  be  turned  towards  the  earth,  and 
she  will  then  appear  horned  as  at  6;  when  she  has  per- 


ON  THE  MOON.  165 

formed  one  quarter  of  her  orbit,  she  shows  usone  half  of 
her  enlightened  side  as  at  c  ;  aid  she  is  said  to  be  gib- 
bous, and  at  e  the  whole  of  t!ie  enlightened  side  appears 
to  us,  and  the  moon  is  at  full.  As  she  proceeds  in  her 
orbit  she  becomes  again  gibbous,  and  her  enliijhfened 
hemisphere  turns  gradually  away  from  us  until  she 
completes  her  orbit  and  disappears,  and  then  again  re- 
sumes her  form  of  a  new  moon. 

When  the  moon  is  at  full,  or  a  new  moon,  she  is  said 
to  be  in  conjunction  with  the  sun,  as  they  are  then  both 
in  the  same  direction  with  regard  to  the  earth  ;  when  at 
her  quarters  she  is  said  to  be  in  opposition  to  the  sun. 

Emily.  Are  not  the  eclipses  produced  by  the  moon 
passing  between  the  sun  and  the  earth  ? 

Mrs,  B,  Yes;  when  the  moon  passes  between  the 
sun  and  the  earth,  she  intercepts  his  rays,  or  in  other 
words,  casts  a  shadow  on  the  earth,  then  the  sun  is 
eclipsed,  and  the  day  light  gives  place  to  darkness, 
while  the  moon's  shadow  is  passing  over  us. 

When,  on  the  contrary,  the  earth  is  between  the  sun 
and  the  moon,  it  is  we  who  intercept  the  sun's  rays,  and 
cast  a  shadow  on  the  moon ;  the  moon  is  then  darkened, 
she  disappears  from  our  view,  and  is  eclipsed. 

Emily.  But  as  the  moon  goes  round  the  earth  every 
month,  she  must  be  once/luring  that  time  between  the 
earth  and  the  sun,  and  the  earth  must  likewise  be  once 
between  the  sun  and  the  moon,  and  y^i  we  have  not  a 
solar  and  a  lunar  eclipse  every  month  ? 

Mrs.  B,  The  orbits  of  the  earth  and  moon  are  not 
exactly  parallel,  but  cross  or  intersect  each  other  ;  and 
the  moon  generally  passes  either  above  or  below  the 
earth  when  she  is  in  conjunction  with  the  sun,  and  does 


166  ON  THE  MOON. 

therefore  intercept  the  sun's  rays,  and  produce  an 
eclipse;  for  this  can  take  place  only  when  the  earth 
and  moon  are  in  conjunction  in  that  part  of  their  orbits 
which  cross  each  other,  (called  the  nodes  of  their  orbits) 
because  it  is  then  only,  that  they  are  both  in  a  right  line 
with  the  sun. 

Emili/,  And  a  partial  eclipse  takes  place,  I  suppose, 
when  the  moon  in  pasj^ing  by  the  earth,  is  not  suffici- 
ently above  or  below  the  earth's  shadow  entirely  to 
escape  it  ? 

Mrs,  B,  Yes,  one  edge  of  h^r  disc  then  dips  into  the 
shadow,  and  is  ecliped ;  but  as  the  earth  is  larger  than 
the  moon,  when  the  eclipse  happens  precisely  at  the 
nodes,  they  are  not  only  total,  but  last  for  some  length 
of  time. 

When  the  sun  is  eclipsed,  the  tofal  darkness  is  con- 
fined to  one  particular  part  of  the  earth,  evidently 
showing  that  the  moon  is  smaller  than  the  earth,  since 
she  cannot  entirely  skreen  it  from  the  sun.  In  fig.  1,  , 
pi.  XII.  you  will  find  a  solar  eclipse  described.;  S  is  the 
sun,  M  the  moon,  and  E  tlie  earth;  and  the  moon's 
shadow,  you  see,  is  not  large  enough  to  cover  the  earth. 
The  lunar  eclipses  on  the  contrary  are  visible  from  every 
part  of  the  earth,  where  the  moon  is  above  the  horizon; 
and  we  discover  by  the  length  of  time  which  the  moon 
is  in  passing  through  the  earth's  shadow,  that  it  would 
be  sufficient  to  eclipse  her  totally,  were  she  47  times 
her  actual  size  ;  it  follows  therefore,  that  the  earth  is 
47  times  the  size  of  the  moon. 

In  fig.  2.  S  represents  the  sun,  which  pours  forth 
rays  of  lig]>t  in  stiaight  lines  in  every  direction.  E  is 
the  earth,  and  M  the  moon.    Now  a  ray  of  light  coming 


/'ul>:ijy  ./.YlhmO>/'ny,   I'/n/.ni  .' 


ON  THE  MOON.  167 

from  one  extremity  of  the  sun's  disc  in  the  direction 
A  B,  will  meet  another  coming  from  the  opposite  extre- 
mity in  the  direction  C  B  ;  the  shadow  of  the  earth 
cannot  therefore  extend  beyond  B  ;  as  the  sun  is  larger 
than  the  earth,  the  shadow  of  the  latter  is  conical,  or 
the  figure  of  a  sugar  loaf;  it  gradually  diminishes,  and 
is  much  smaller  than  the  earth'where  the  moon  passes 
through  it,  and  yet  we  find  the  moon  to  be  not  only  to- 
tally eclipsed,  but  some  length  of  time  in  darkness, 
and  hence  we  are  enabled  to  ascertain  its  real  di- 
mensions. 

Emily,  When  the  moon  eclipses  the  sun  to  us,  we 
must  be  eclipsed  to  the  moon  ? 

Mrs.  B.  Certainly ;  for  if  the  moon  intercepts  the 
sun's  rays,  and  cast  a  shadow  on  us,  we  must  necessarily 
disappear  to  the  moon,  but  only  partially,  as  in  fig.  1. 

Caroline,  There  must  be  a  great  number  of  eclipses 
1n  the  distant  planets,  which  have  so  many  moons  ? 

Mrs,  B,  Yes,  few  days  pass  without  an  eclipse  taking 
place ;  for  among  the  number  of  satellites,  one  or  the 
other  of  them  are  continually  passing  either  between 
their  planet  and  the  sun,  or  between  the  planet  and 
each  other.  Astronomers  are  so  well  acquainted  with 
the  motion  of  the  planets  and  their  satellites,  that  they 
liave  calculated  not  only  the  eclipses  of  our  moon,  but 
those  of  Jupiter,  with  such  perfect  accuracy,  that  it  has 
afforded  a  means  of  ascertaining  the  longitude. 

Caroline,  But  is  it  not  very  easy  to  find  both  the 
latitude  and  longitude  of  any  place  by  a  map  or  globe? 

Mrs,  B,  If  you  know  where  you  are  situated,  there 
is  no  difficulty  in  ascertaining  the  latitude  or  longitude 
of  the  place  by  referring  to  a  map  ;  but  supposing  that 


168  ON  THE  MOON. 

you  had  been  a  length  of  time  at  sea,  interrupted  in 
your  course  by  storms,  a  map  would  afford  you  very 
little  assistance  in  discovering  where  you  were. 

Caroline,  Under  such  circumstances,  I  confess  I 
should  be  equally  at  a  loss  to  discover  either  latitude  or 
longitude. 

Mrs.  B,  The  latitude  may  be  easily  found  by  taking 
the  altitude  of  the  pole  ;  that  is  to  say,  the  number  of  de- 
grees that  it  is  elevated  above  the  horizon,  for  the  pole 
appears  more  elevated  as  we  approach  it,  and  less  as 
we  recede  from  it. 

Caroline,  But  unless  you  can  see  the  pole  how  can 
you  take  its  altitude  ? 

Mrs,  B.  The  north  pole  points  constantly  towards 
one  particular  part  of  the  heavens  in  which  a  star  is  si- 
tuated, called  the  Polar  Star:  this  star  is  visible  on 
clear  nights,  from  every  part  of  the  northern  hemis- 
phere, the  altitude  of  the  polar  star,  is  therefore  the 
same  number  of  degrees  as  that  of  the  pole ;  the  lati- 
tude may  also  be  determined  by  observations  made  on 
the  sun  or  any  of  the  fixed  stars;  the  situation  tlierefore 
of  a  vessel  at  sea,  with  regard  to  north  and  south,  is 
easily  ascertained.  The  difficulty  ib  respecting  east  and 
west,  that  is  to  say  its  longitude.  As  we  have  no  east- 
ern poles  from  which  we  can  reckon  our  distance;  some 
particular  spot  must  be  fixed  upon  for  that  purpose. 
The  English  reckon  from  the  meridian  of  Greenwich, 
where  the  royal  observatory  is  situated ;  in  French  maps 
you  will  find  that  the  lon^^itude  is  reckoned  from  Paris. 

The  rotation  of  the  earth  on  its  axis  in  24  hours 
from  west  in  east  occasions,  you  know,  an  apparent 
motion  of  the  sun  and  stars  in  the  contrary  directioo. 


ON  THE  MOON.  169 

"and  the  sun  appears  to  go  round  the  earth  in  the  space  of 
24  hours,  passing  over  fifteen  degrees  or  a  twenty-fourth 
part  of  the  earth's  circumference  every  hour ;  therefore 
when  it  is  twelve  o'clock  in  London,  it  is  one  o'clock  in 
any  place  situated  fifteen  degrees  to  the  east  of  London, 
as  the  sun  must  have  passed  the  meridian  of  that  place  aa 
hour  before  he  reaches  that  of  London.  For  the  same 
reason  it  is  eleven  o'clock  to  any  place  situated  fifteen 
degrees  to  the  west  of  London,  as  the  sun  will  not  come 
to  that  meridian  till  an  hour  later. 

If  then  the  captain  of  a  vessel  at  sea,  could  know 
precisely  what  was  the  hour  at  London,  he  could,  by 
lookiilg  at  his  watch,  and  comparing  it  with  the  hour  of 
the  spot  in  which  he  was,  ascertain  the  longitude. 

Emily,  But  if  he  had  not  altered  his  watch,  since 
he  sailed  from  London,  it  would  indicate  the  hour  it  was 
then  in  London. 

Mrs,  B,  True ;  but  in  order  to  know  the  hour  of 
the  day  of  the  spot  in  which  he  is,  the  captain  of  a  ves- 
sel regulates  his  watch  by  the  sun  when  it  reaches  the 
meridian. 

Emily,  Then  if  he  had  two  watches,  he  might  keep 
one  regulated  daily, and  leave  the  other  unaltered;  the 
former  would  -indicate  the  hour  of  the  place  in  which 
he  was  situated,  and  the  latter  the  hour  of  London ; 
and  by  comparing  them  together,  he  would  be  able  to 
calculate  his  longitude. 

Mrs,  B,  You  have  discovered,  Emily,  a  mode  of 
finding  the  longitude,  which  I  have  the  pleasure  to  tell 
you,  is  universally  adopted  :  watches  of  a  superior  con- 
struction, called  chronometers,  or  time-keepers,  are  used 
for  this  pui-pose  ;  but  the  best  watches  are  liable  to  ira^ 


irO  ON  THE  MOON. 

perfections,  and  should  the  time-keeper  go  too  fast,  or 
too  slow,  thf»re  would  he  no  means  of  ascertaining  the 
error;  implicit  reliance  cannot  consequently  be  placed 
upon  them. 

Recourse  is  therefore  had  to  the  eclipses  of  Jupiter's 
satellites.  A  table  is  made  of  the  precise  time  at  which 
the  several  moons  are  eclipsed  to  a  spectator  at  Lcmdon; 
when  they  appear  eclipsed  to  a  spectator  in  any  other 
spot,  he  may,  by  consulting  the  table,  know  what  is  the 
hour  at  London  ;  for  the  eclipse  is  visible  at  the  same 
moment  from  whatever  place  on  the  earth  it  is  seen.  He 
has  then  only  to  look  at  the  watch  which  points  out  the 
hour  of  the  place  in  which  he  is,  and  by  observing  the 
difference  of  time  there,  and  at  London,  he  may  imme- 
diately determine  his  longitude. 

Let  us -suppose,  that  a  certain  moon  of  Jupiter  is  al- 
ways eclipsed  at  six  o'clock  in  the  evening ;  and  that 
a  man  at  sea  consults  his  watch,  and  finds  that  it  is  ten 
o'clock,  at  night,  where  he  is  situated,  at  the  moment 
the  eclipse  takes  place  ;  what  will  be  his  longitude  ? 

Emily.  That  is  four  hours  later  than  in  London ; 
four  times  fifteen  degrees  make  60  ;  he  would,  there- 
fore, be  sixty  degrees  east  of  London,  for  the  sun  must 
have  passed  his  meridian  before  it  reaches  that  of 
London. 

JJrs,  B,  For  this  reason  the  hour  is  always  later 
than  in  Londbn,  when  the  place  is  east  longitude,  and 
earlier  when  it  is  west  longitude.  Thus  ,the  longitude 
can  be  ascertained  whenever  the  eclipses  of  Jupiter's 
moon's  are  visible. 

But  it  is  not  only  the  secondary  planets  which  produce 
eclipses,  for  the  primary  planets  near  the  sun  eclipse 


ON  THE  MOON.  171 

him  to  those  at  a  ^^reater  distance  when  thej  come  in 
conjunction  in  the  nodes  of  their  orbits  ;  but  as  the 
primary  planets  are  much  longer  in  performing  their 
course  round  the  sun,  than  the  satellites  in  going  round 
ilieir  primary  planets,  these  eclipses  very  seldom  occur. 
Mercury  and  Venus  have  however  passed  in  a  right  line 
between  us  and  the  sun,  but  being  at  so  great  a  distance 
from  us,  their  shadows  did  not  extend  so  far  as  the 
earth  ;  no  darkness  was  therefore  produced  on  any  part 
of  our  globe  ;  but  the  planet  appeared  like  a  sjnall 
black  spot,  passing  across  the  sun's  disc  :  this  is  called 
•a  transit  of  the  planet. 

It  was  by  the  last  transit  of  Venus,  that  astronomers 
were  enabled  to  calculate  with  some  degree  of  accuracy 
the  distance  of  the  earth  from  the  sun,  and  the  dimen- 
sions of  the  latter. 

Emily.  I  have  heard  that  the  fides  arp  affected  by 
ihe  moon,  but  I  cannot  conceive  what  influence  it  can 
have  on  them. 

Mr^s,  B,  They  are  produced  by  the  moon's  attrac- 
tion, which  draws  up  the  waters  in  a  protuberance. 

Caroline.  Does  attraction  act  on  water  more  power- 
fully than  on  land  ?  I  should  have  thought  it  would  have 
been  just  the  contrary,  for  land  is  certainly  a  more 
dense  body  than  water  ? 

Mrs,  B.  Tides  do  not  arise  from  water  being  more 
strongly  attracted  than  land,  for  this  certainly  is  not 
the  case  ;  but  the  cohesion  of  fluids  being  much  less 
than  that  of  solid  bodies,  they  more  easily  yield  to  the 
power  of  gravity,  in  consequence  of  which  the  waters 
immediately  below  the  moon  are  drawn  up  by  it  in  a 
protuberance,  producing  a  full  tide,  or  what  is  common- 


172  ON  THE  MOON. 

Ij  called  high  water,  at  the  spot-  where  it  happens.  So 
far  the  theory  of  the  tides  is  not  difficult  to  understand. 

Caroline.  On  the  contrary,  nothing  can  be  more 
simple  ;  the  waters,  in  order  to  rise  up  under  the  moon, 
must  draw  the  waters  from  the  opposite  side  of  the  globe» 
and  occasion  ebb-tide,  or  low  water  in  thos6  parts. 

Mrs,  B,  You  draw  your  conclusion  rather  too  hasti- 
ly my  dear ;  for  according  to  your  theory,  we  should 
bave  full  tide  only  once  in  twenty-four  hours,  that  is, 
every  time  tliat  we  were  below  the  moon,  while  we  find 
that  we  have  two  tides  in  the  course  of  twenty-four 
hours,  and  that  it  is  high-water  with  us  and  with  our  an- 
tipodes at  the  same  time. 

Caroline,  Yet  it  must  be  impossible  for  the  moon  to 
fittract  the  sea  in  opposite  parts  of  the  globe,  and  in  op- 
posite directions  at  the  same  time. 

Mrs,  B*  This  opposite  tide  is  rather  more  difficult 
to  explain,  than  that  which  is  drawn  up  beneath  the 
moon  ;  with  a  little  attention,  however,  I  hope  I  shall 
be  able  to  make  you  understand  it. 

You  recollect  that  the  earth  and  moon  are  mutually 
attracted  towards  a  point,  their  common  centre  of  gra^ 
vity  and  of  motion ;  can  you  tell  me  what  it  is  that 
prevents  their  meeting  and  uniting  at  this  point  ? 

Emily,  Their  projectile  force,  which  gives  them  a 
tendency  to  fly  from  this  centre. 

Mrs,  B,  And  is  hence  called  their  centrifugal  force. 
Now  we  know  that  the  centrifugal  force  increases  in 
proportion  to  the  distance  from  the  centre  of  motion. 

Caroline,  Yes,  1  recollect  your  explaining  that  to 
us,  and  illustrating  it  by  the  motion  of  the  flyers  of  a 
wind-mill,  and  the  spinning  of  a  top. 


ON  THE  MOON.  173 

Emily.  And  it  was  but  the  other  day  you  showed  us 
that  bodies  weighed  less  at  the  equator,  than  in  the  polar 
regions,  in  consequence  of  the  increased  ^centrifugal 
force  in  the  equatorial  parts. 

Mrs,  B.  Very  well.  The  power  of  attraction,  on 
the  contrary,  increases  as  the  distance  from  the  centre 
of  gravity  diminishes  ;  when,  therefore,  the  two  centres 
of  gravity  and  of  motion  are  in  the  same  spot,  as  is  the 
case  with  regard  to  the  moon  and  the  earth,  the  centri- 
fugal power  and  those  of  attraction,  will  be  in  inverse 
proportion  to  each  other  ;that  is  to  say,  where  the  one  is 
strongest,  the  other  will  be  w^eakest. 

Emibj.  Those  parts  of  the  ocean,  then,  which  are 
most  strongly  attracted  will  have  least  centrifugal  force ; 
and  those  parts  which  are  least  attracted,  will  have 
the  greatest  centrifugal  force. 

Mrs.  B.  In  order  to  render  the  question  more  sim- 
ple, let  us  suppose  the  earth  to  be  every  where  covered 
by  the  ocean,  as  represented  in  (fig.  3.  pi.  Xll.)  M  is 
the  moon,  A  B  C  D  the  earth,  and  X  the  common  ceiitre 
of  gravity  and  of  motion  of  these  two  planets.  Now 
the  waters  on  the  surface  of  the  earth,  about  A,  being 
more  strongly  attracted  than  any  other  part,  will  be  ele- 
vated ;  the  attraction  of  the  moon  at  B  and  C  being 
less,  and  at  D  least  of  all.  But  the  centrifugal  force 
at  D,  will  be  greatest,  and  the  waters  there,  will  in  con- 
sequence have  the  greatest  tendency  to  recede  from  the 
moon  ;  the  waters  at  B  and  C  will  have  less  tendency 
to  recede,  and  at  A  least  of  all.  The  waters,  therefore, 
at  D,  will  recede  furthest,  at  the  same  time  that  they 
are  least  attracted,  and  in  consequence  will  be  elevated 
la  a  protuberaenc  similar  to  that  at  A. 
p  2 


174  ON  THE  MOON. 

Emily,  The  tide  A,  then,  is  produced  by  the  moon's 
attraction,  and  increased  by  the  feebleness  of  the  centri- 
fugal power  in  those  parts  ;  and  the  tide  D  is  produced 
by  the  centrifugal  force,  and  increased  by  the  feeble- 
ness of  the  moon's  attraction  in  those  parts. 

Caroline,  And  when  it  is  high  water  at  A  and  D,  it 
is  low  water  at  B  and  C  :  now  I  think  1  comprehend  the 
nature  of  the  tides  again,  though  I  confess  it  is  not 
quite  so  easy  as  I  at  first  thought. 

But,  Mrs.  B.,  why  does  not  the  sun  produce  tides 
as  well  as  the  moon  ;  for  its  attraction  is  greater  than 
that  of  the  moon  ? 

Mrs,  B,  It  would  be  at  an  equal  distance,  but  our 
vicinity  to  the  moon  makes  her  influence  more  pow- 
erful. The  sun  has  however,  a  considerable  effect  on 
the  tides,  and  increases  or  diminishes  them  as  it  acts 
in  conjunction  with,  or  in  opposition  to  the  moon. 
Emily,  I  do  not  quite  understand  that. 
Mrs,  B,  The  moon  is  a  month  in  going  round  the  earth; 
twice  during  that  time,  therefore,  at  full  and  at  change, 
she  is  in  the  same  direction  as  the  sun,  both  then  act  in 
conjunction  on  the  earth,  and  produce  very  great  tides, 
called  spring  tides,  as  described  in  fig.  4,  at  A  and  B; 
but  when  the  moon  is  at  the  intermediate  parts  of  her 
orbit,  the  sun,  instead  of  aftbrding  assistance,  weakens 
her  power  by  acting  in  opposition  to  it ;  and  smaller 
tides  are  produced,  called  neap  tides,  as  represented  in 
fig.  5. 

Emily,  I  have  often  observed  the  difference  of  these 
tides  when  I  have  been  at  the  sea  side. 

But  since  attraction  is  mutual  between  the  moon  and 
the  earth,  we  must  produce  tides  in  the  moon ;  and  these 


ON  THE  MOON.  175 

must  be  more  considerable  in  proportion  as  our  planet 
is  larger.  And  jet  the  moon  does  not  appear  of  an 
oval  form, 

Mrs.  B.  You  must  recollect,  that  in  order  to  render 
the  explanation  of  the  tides  clearer,  we  suppose  the 
whole  surface  of  the  earth  to  be  covered  with  the  ocean  ; 
but  that  is  not  really  the  case,  either  with  the  earth  or  the 
moon,  and  the  land  which  intersects  the  water  destroys 
the  regularity  of  the  effect. 

Caroline,  True  ;  we,  may  however  be  certain,  that 
whenever  it  is  high  water  the  moon  is  immediately 
over  our  heads. 

Mrs,  B,  Not  so  either ;  for  as  a  similar  effect  is 
produced  on  that  part  of  the  globe  immediately  beneath 
the  moon,  and  on  that  part  most  distant  from  it,  it  can- 
not be  over  the  heads  of  the  inhabitants  of  both  those 
situations  at  the  same  time.  Besides,  as  the  orbit  of 
the  moon  is  very  nearly  parallel  to  that  of  the  earth, 
she  is  never  vertical  but  to  the  inhabitants  of  the  torrid 
zone  ;  in  that  climate,  therefore,  the  tides  are  greatest 
and  they  diminish  as  you  recede  from  it  and  approach 
the  poles. 

Caroline.  In  the  torrid  zone,  then,  I  hope  you  will 
grant  that  the  moon  is  immediately  over,  or  opposite 
the  spots  where  it  is  high  water  ? 

Mrs.  B.  1  cannot  even  admit  that;  for  the  ocean 
naturally  partaking  of  the  earth's  motion,  in  its  rota- 
tion from  west  to  east,  the  moon,  in  forming  a  tide,  has 
to  contend  against  the  eastern  motion  of  the  waves.  All 
matter,  you  know,  by  its  inertia,  makes  some  resistance 
to  a  change  of  state  ;  the  waters,  therefore,  do  not  rea- 
dily yield  to  the  attraction  of  the  moon,  and  the  effect 


176  ON  THE  MOON. 

of  her  influence  is  not  complete  till  three  hours  after 
she  has  passed  the  meridian,  where  it  is  full   tide. 

Emily.  Pray  what  is  the  reason  that  the  tide  is 
three-quarters  of  an  hour  later  every  day  ? 

Mrs,  B.  Because  it  is  twenty-four  hours  and  three- 
quarters  before  the  same  meridian  on  our  globe  returns 
beneath  the  moon.  The  earth  revolves  on  its  axis  in 
about  twenty-four  hours ;  if  the  moon  were  stationary, 
therefore,  the  same  part  of  our  globe  would,  every  twen- 
ty-four hours,  return  beneath  tJie  moon  ;  but  as  during 
our  daily  revolution  the  moon  advances  in  her  orbit, 
the  earth  must  make  more  than  a  complete  rotation  in 
order  to  bring  the  same  meridian  opposite  the  moon  : 
we  are  three-quarters  of  an  hour  in  overtaking  her.  The 
tides,  therefore,  are  retarded  for  the  same  reason  that  the 
moon  rises  later  by  three-quarters  of  an  hour  everyday. 

Weliave  now,  I  think,  concluded  the  observations  I 
had  to  make  to  you  on  the  subject  of  astronomy ;  at  our 
next  interview,  1  shall  attempt  to  explain  to  you  the 
elements  of  hydrostatics. 


'mM^^'^ 


CONVERSATION  X. 


ON  THE  MECHANICAL  PROPERTIES  OV 
FLUIDS.  - 

DEFJMTIO?^     or    A    TLVID DISTIXCTION     BETWEEN     FLUIDS     AND 

Liat'IDS. — OF      NON-ELASTIC       FLUIDS SCARCELY      SUSCEPTIBLK 

OF    COMPRESSION. — OF    THE     COHESION     OF     FLUIDS. OF     THEltt 

GRAVITATION. OF    THEIR  EQ,UILIBRIUM. — OF  THEIR  PXESSURE 

—OF  SPECIFIC  GRAVITY. OF  THE  SPECIFIC  GRAVITY  OF  BO- 
DIES HEAVIER  THAN  WATER.-^OF  THOSE  OF  THE  SAME  WEIGHT 
AS  WATER. OF  THOSE  LIGHTER  THAN  WATER. OF  THE  SPE- 
CIFIC   GRAVITY    OF    FLUIDS. 


Mrs.  B. 

We  have  hitherto  confined  our  attention  to  the  me- 
chanical properties  of  solid  bodies,  which  have  been  il- 
lustrated, and,  I  hope,  thoroughly  impressed  upon  your 
memory,  by  the  conversations  we  have  subsequently 
had  on  astronomy.  It  will  now  be  necessary  for  me  to 
give  you  some  account  of  the  mechanical  properties  of 
fluids — a  science  which  is  called  hydrostatics.  A  fluid 
is  a  substance  which  yields  to  the  slightest  pressure.     If 


178  MECHANICAL  PROPERTIES  OF  FLUIBS. 

you  dip  jour  hand  into  a  basin  of  water,  jou  are  scareie- 
Ij  sensible  of  meeting  with  any  resistance. 

Emily.  The  attraction  of  cohesion  is  then,  I  suppose, 
less  powerful  in  fluids  than  in  solids  ? 

Mrs,  B.  Yes  ;  fluids,  generally  speaking,  are  bodies 
of  less  density  than  solids.  From  the  slight  cohesion, 
of  the  particles  of  fluids,  and  the  facility  with  which 
they  slide  over  each  other,  it  is  inferred,  that  they  must 
be  small,  smooth,  and  globular;  smooth,  because  there 
appears  to  be  little  or  no  friction  among  them  ;  and  glo- 
bular, because  touching  each  other  but  by  a  point  would 
account  for  the  slightness  of  their  cohesion. 

Caroline,  Pray  what  is  the  distinction  between  a 
fluid  and  a  liquid? 

Mrs,  B,  Liquids  comprehend  only  one  class  of 
fluids.  There  is  another  class  distinguished  by  the 
name  of  elastic  fluids,  or  gases,  which  comprehends  the 
air  of  the  atmosphere,  and  all  the  various  kinds  of  air 
with  which  you  will  become  acquainted  when  you  study 
chemistry.  Their  mechanical  properties  we  shall  ex- 
amine at  our  next  meeting,  and  confine  our  attention 
this  morning  to  those  of  liquids,  or  non-elastic  fluids. 

Water,  and  liquids  in  general,  are  scarcely  suscepti- 
ble of  being  compressed,  or  squeezed  into  a  smaller 
space  than  that  which  they  naturally  occupy.  This  is 
supposed  to  be  owing  to  the  extreme  minuteness  of  their 
particles,  which,  rather  than  submit  to  compression, 
force  their  way  through  the  pores  of  the  substance 
which  confines  them.  This  was  shown  by  a  celebrated 
experiment,  made  at  Florence  many  years  ago.  A  hol- 
low globe  of  gold  was  filled  with  water,  and  on  its  be- 
ing submitted  to  great  pressure,  the  water  was  seen  to 


MECHANICAL  PROPERTIES  OF  FLUIDS.        179 

exude  through  the  pores  of  the  gold,  which  it  covered 
with  a  fine  dew.  Fluids  gravitate  in  a  more  perfect 
manner  than  solid  bodies  ;  for  the  strong  cohesive  at- 
traction of  the  particles  of  the  latter  in  some  measure 
counteracts  the  effect  of  gravity.  In  this  table,  for  in- 
stance, the  cohesion  of  the  particles  of  wood  enables 
four  slender  legs  to  support  a  considerable  weight. 
Were  the  cohesion  destroyed,  or,  in  other  words,  the 
wood  converted  into  a  fluid,  no  support  could  be  af- 
forded bv  the  legs,  for  the  particles  no  longer  cohering 
together,  each  would  press  separately  and  indepen- 
dently, and  w  ould  be  brought  to  a  level  with  the  sur- 
face ot  the  earth. 

Emily,  This  want  of  cohesion  is  then  the  reason 
why  fluids  can  never  be  formed  into  figures,  or  main- 
tained in  heaps  ;  for  though  it  is  true  the  wind  raises 
water  into  waves,  they  are  immediately  afterwards 
destroyed  by  gravity,  and  water  always  finds  its  level. 

Mrs.  B.  Do  you  understand  what  is  meant  by  the 
level,  or  equilibrium  of  fluids  ? 

Emily,  I  believe  I  do, .though  I  feel  rather  at  a  loss 
to  explain  it.  Is  not  a  fluid  level  when  its  sorface  is 
smooth  and  flat,  as  is  the  case  with  all  fluids  when  in 
a  state  of  rest  ? 

Mrs,  B.  Smooth,  if  you  please,  but  not  flat;  for  the 
definition  of  the  equilibrium  of  a  fluid  is,  that  every 
part  of  the  surface  is  equally  distant  from  the  point  to 
which  gravity  tends,  that  is  to  say,  from  the  centre  of 
the  earth  ;  hence  the  surface  of  all  fluids  must  be  bulg- 
ing, not  flat,  since  they  will  partake  of  the  spherical 
form  of  the  globe.  This  is  very  evident  in  large  bodies 
of  water,  such  as  the  ocean,  but  the  sjskericity  of  small 


180        MECHANICAIi  PROPERTIES  OF  FLUIDS. 

bodies  of  water  is  so  trifling,  that  their  surfaces  appear 
flat. 

This  level,  or  efjuilibrium  of  fluids,  is  the  natural 
result  of  their  particles  graviiating  independently  of 
each  other  ;  for  when  any  particle  of  a  fluid  acciden- 
tally finds  itself  elevated  above  the  rest,  it  is  attracted 
down  to  the  level  of  the  surface  of  the  fluid,  and  the 
readinev«s  with  which  fluids  yield  to  the  slightest  im- 
pression, will  enable  the  particle  by  its  weight  to  pen- 
etrate the  surface  of  the  fluid  and  mix  with  it. 

Caroline.  But  I  have  seen  a  drop  of  oil  float  on  the 
surface  of  water  without  mixing  with  it. 

Mrs,  B,  That  is,  because  oil  is  a  lighter  liquid  than 
water.  If  you  were  to  pour  water  over  it,  the  oil  would 
rise  to  the  surface,  beilig  forced  up  by  the  superior  gra- 
vity of  the  water.  Here  is  an  instrument  called  a  v\atfr- 
level,  (fig.  1.  plate  XIIL'i  which  is  constructed  upon  the 
principle  of  the  equilibrium  of  fluids.  It  consists  of  a 
short  tube,  A  B,  closed  at  both  ends,  and  containing  a 
little  water;  when  the  tube  is  not  perfectly  horizontal 
the  water  runs  to  the  lower  end,  and  it  is  by  this  means 
that  the  level  of  any  situation,  to  which  we  apply  the 
instrument,  is  ascertained. 

Solid  bodies  you  m  ly,  therefore  consider  as  gravita- 
ting in  masses,  for  the  strong  cohesion  of  their  parti- 
cles makes  them  weigh  altogether,  while  every  particle 
of  a  fluid  may  be  considered  as  composing  a  separate 
mass,  gravitating  independently  of  each  other.  Hence 
the  resistance  of  a  fluid  is  considerably  less  than  that 
of  a  solid  body  ;  for  the  resistance  of  the  particles  act- 
ing separately,  they  are  more  easily  overcome. 


Pl.ATEAlU. 


Fi^.3. 


A 

n ' 

j'y.  I. 

B 

' 

Fifl.2. 


Pi  J),  hv  J.  Y.Ifwixphreys   Thilad. ." 


MECHANICAL  PROPERTIES  OF  FLUIDS.         181 

Emily,  A  body  of  water,  in  falling,  does  certainly 
less  injury  than  a  solid  body  of  the  same  weight. 

Mrs.  B.  The  particles  of  fluids  acting  thus  inde- 
pendently, press  against  each  other  in  every  direction, 
not  only  downwards  but  upwards,  and  laterally  or  side- 
ways ;  and  in  consequence  of  this  equality  of  pres- 
sure, every  particle  remains  at  rest  in  the  fluid.  If  you 
agitate  the  fluid  you  disturb  this  equality  of  pressure 
and  the  fluid  will  not  rest  till  its  equilibrium  is  re- 
stored. 

Caroline,  The  pressure  downwards  is  very  natural ; 
it  is  the  effect  of  gravity,  one  particle  weighing  upon 
another  presses  on  it;  but  the  pressure  sideways,  and 
particularly  the  pressure  upwards,  I  cannot  under- 
stand. 

Mrs.  B.  If  there  were  no  lateral  pressure,  water 
would  not  run  out  of  an  opening  on  the  side  of  a  vessel. 
If  you  till  a  vessel  with  sand,  it  will  not  run  out  of  such 
an  opening,  because  there  is  scarcely  any  lateral  pres- 
sure among  its  particles. 

Emily.  When  water  runs  out  of  the  side  of  a  ves- 
sel, is  it  not  owing  to  the  weight  of  the  water  above  the 
opening.^ 

Mrs.  B.  If  the  particles  of  fluids  were  arranged 
in  regular  columns  thus,  (fig.  2.)  there  would  be  no  la- 
teral pressure,  for  when  one  particle  is  perpendicularly 
above  the  other,  it  can  only  preSs  it  downwards ;  but 
as  it  must  continually  happen,  that  a  particle  presses  be- 
tween two  particles  beneath,  (fig.  3.)  these  last  must 
suffer  a  lateral  pressure. 

Emily.  The  same  as  when  a  w^dge  is  driven  into^ 
a  piece  of  wood,  and  separates  the  parts  laterally. 


182        MECHANICAL  PROPERTIES  OF  FLUIDS. 

Mrs,  B.  Yes.  The  lateral  pressure  proceeds,  there- 
fore, entirely  from  the  pressure  downwards,  or  the 
Weight  of  the  liquid  above  ;  and  consequently  the  low- 
er the  orijBce  is  made  in  the  vessel,  the  j^reater  will  be 
the  velocity  of  the  water  rushing  out  of  it.  Here  is 
«  vessel  of  water  (fig.  4.),  with  three  stop  cocks  at  dif- 
ferent heights  ;  v\e  shall  open  them,  and  you  will  see 
with  what  different  degrees  of  velocity  the  water  issues 
from  them.     Do  you  understand  this,  Caroline  ? 

Caroline,  Oh  yes.  The  water  from  the  upper  spout 
receiving  but  a  slight  pressure,  on  account  of  its  vi- 
cinity to  the  surface,  flows  but  gently  ;  the  second  cock 
having  a  greater  weight  above  it,  the  water  is  forced  out 
with  greater  velocity,  whilst  the  lowest  cock  being  near 
the  bottom  of  the  vessel,  receives  the  pressure  of  al- 
most the  whole  body  of  water,  and  rushes  out  with  the 
greatest  impetuosity. 

Mrs.  B,  Very  well ;  and  you  must  observe,  that  as 
the  lateral  pressure  is  entirely  owing  to  the  pressure 
dow^nwa'ds,  it  is  not  effected  by  the  horizontal  dimen- 
sions of  the  vessel,  which  contains  the  water,  but  iirf  re-^ 
ly  by  its  depth  ;  for  as  every  particle  acts  independently 
of  the  rest,  it  is  only  the  column  of  particles,  imme- 
diately above  the  orifice  that  can  weigh  upon  and  press 
out  the  water. 

Emily,  The  breadth  and  width  of  the  vessel  then 
can  be  of  no  consequence  in  this  respect.  The  lateral 
pressure  on  one  side,  in  a  cubical  vessel,  is,  1  suppose 
not  so  great  as  the  pressure  downwards. 

Mrs,  B,  No ;  in  a  cubical  vessel  the  pressure  down- 
wards v*^ill  be  double  the  lateral  pressure  on  one  side; 
for  every  particle  at  the  bottom  of  the  vessel  is  pressed 


MECHANICAL  PROPERTIES  OF  FLUIDS.         183 

«pon  bj  a  column  of  the  whole  depth  of  the  fluid,  whilst 
the  lateral  pressure  diminishes  from  the  bottom  upwards 
to  the  surface,  where  the  particles  have  no  pressure. 

Caroline,  And  from  whence  proceeds  the  pressure 
of  fluids  upwards?  that  seems  to  me  the  most  unac- 
couijtable,  as  it  is  in  direct  opposition  to  gravity. 

Mrs.  B,  And  yet  it  is  a  consequence  of  their  pressure 
downwards.  When,  for  example,  you  pour  water  into 
a  tea-pot,  t!ie  water  rises  in  the  spout  to  a  level  witli 
the  water  in  the  pot.  The  particles  of  water  at  the 
bottom  of  the  pot  are  pressed  upon  by  the  particles 
above  them  ;  to  this  pressure  they  will  yield,  if  there 
is  any  mode  of  making  way  for  the  -^u-jerior  particles, 
and  as  they  cannot  descend,  they  will  change  their  di- 
rection and  rise  in  the  spout. 

Suppose  the  tea-pot  to  be  filled  with  columns  of  par- 
ticles of  water  similar  to  that  described  in  fig.  4.  the 
particle  1  at  the  bottom  will  be  pressed  laterally  by  the 
particle  2,  and  by  this  pressure  be  forced  into  the  spout, 
where  meeting  with  the  particle  3,  it  presses  it  upwards 
and  this  pressure  will  be  continued,  from  3  to  4,  from  4 
to  5,  and  so  on  till  the  water  in  the  spout  has  risen  to 
a  level  with  that  in  the  pot. 

Emily,  If  it  were  not  for  this  pressure  upwards, 
forcing  the  water  to  rise  in  the  spout,  the  equilibrium 
of  the  fluid  would  be  destroyed. 

Caroline,  True  ;  but  then  a  tea-pot  is  wide  and  large, 
and  the  weight  of  so  great  a  body  of  water  as  the  pot 
will  contain,  may  easily  force  up  and  support  so  small 
a  quantity  as  will  fill  the  spout.  But  would  the  same 
effect  be  produced  if  the  spout  and  the  pot  were  of  equal 
dimensions  ? 


184        MECHANICAL  PROPERTIES  OF  FLUIDS. 

Mrs,  B.  Undoubtedly  it  would.  You  may  even 
reverse  the  experiment  by  pouring  wat^r  into  the  spout, 
and  you  will  find  that  the  water  will  rise  in  the  pot  to  a 
level  with  that  in  the  spout;  for  the  pressure  of  the 
su.all  quantity  of  water  in  the  spout  will  force  up  and 
support  thq  larger  quantity  in  the  pot.  In  the  pressure 
upwards,  as  well  as  that  laterally,  you  see  that  the 
force  of  pressure  depends  entirely  on  the  height,  and  is 
quite  independent  of  the  horizontal  dimensions  of  the 
fluid. 

As  a  tea-pot  is  not  transparent,  let  us  try  the  expe- 
riment by  filling  this  large  glass  goblet  by  means  of  this 
narrow  tube,  (fig.  6.) 

Caroline,  Look,  Emily,  as  Mrs.  B.  fills  it,  how  the 
water  ri=c3  in  the  goblet,  to  maintain  an  equilibrium  with 
that  in,  the  tube. 

Now,  Mrs.  B.,  will  you  let  me  fill  the  tube  by  pouring 
water  into  the  goblet  ? 

Mrs,  B,  That  is  impossible.  However,  you  may 
try  the  experiment,  and  I  doubt  not  but  that  you  will 
be  able  to  account  for  its  failure. 

Caroline,  It  is  very  singular,  that  if  so  small  a  co- 
lumn of  water  as  is  contained  in  the  tube  can  force  up 
and  support  the  whole  contents  of  the  goblet;  that  the 
weight  of  all  the  water- in  the  goblet  vshould  not  be  able 
to  force  up  the  small  quantity  required  to  fill  the  tube  : 
—-oh,  I  see  now  the  reason,  the  water  in  the  goblet  can- 
not force  that  in  the  tube  above  its  level,  and  as  the  end 
of  the  tube  is  considerably  higher  than  the  goblet,  it 
can  never  be  filled  by  pouring  water  into  the  goblet. 

Mrs„  B,    And  if  you  continue  to   pour  water  into 


MECHANICAL  PROPERTIES  OF  FLUIDS.        185 

the  goblet  when  it  is  full,  the  water  will  run  over  in- 
stead of  rising  above  the  level  in  the  tube. 

I  shall  now  explain  to  jou  the  meaning  of  the  spe^ 
cific  gravity  of  bodies. 

Caroline.  What !  is  there  another  species  of  gravity 
with  which  we  are  not  yet  acquainted  ? 

Mrs.  B.  No  ;  the  specific  gravity  of  a  body,  means 
simply  its  weight  compared  with  that  of  another  body  of 
the  same  size.  When  we  say,  that  subkances  such  as 
•lead  and  stones  are  heavy,  and  that  others,  such  as  pa- 
per and  feathers,  are  light,  we  speak  comparatively; 
that  is  to  say,  that  the  first  are  heavy,  and  the  latter 
light,  in  comparison  with  the  generality  of  substances 
in  nature.  Would  you  call  wood  and  chalk  light  or 
heavy  bodies  ^ 

Caroline.  Some  kinds  of  wood  are  heavy  certainly, 
as  oak  and  mahogany  ;  others  are  light,  as  deal  and 
box. 

Emily.  I  think  I  should  call  wood  in  general  a  hea- 
vy body,  for  deal  and  box  are  light  only  in  comparison 
to  wood  of  a  heavier  description.  I  am  at  a  loss  to  de- 
termine whether  chalk  should  be  ranked  as  a  heavy  or 
a  light  body ;  I  should  be  inclined  to  say  the  former,  if 
it  was  not  that  it  is  lighter  than  most  other  minerals.  I 
perceive  that  we  have  but  vague  notions  of  light  and 
heavy.  I  wish  there  was  some  standard  of  comparison, 
to  which  we  could  refer  the  weight  of  all  other  bodies. 

Mrs.  B.  The  necessity  of  such  a  standard  has  been 
so  much  felt,  that  a  body  has  been  fixed  upon  for  this 
purpose.  What  substance  do  you  think  would  be  best 
calculated  to  answer  this  end  } 


186        MECHANICAL  PROPERTIES  OF  FLUIDS. 

Caroline.  It  must  be  one  generally  known  and  easi- 
ly obtained,  lead  or  iron  for  instance. 

Mrs,  B.  All  the  metals  expand  by  heat,  and  con- 
dense by  cold.  A*  piece  of  lead,  let  us  say  a  cubi^.  inch 
for  instance,  would  have  less  specific  gravity  in  summer 
than  in  winter ;  for  it  would  be  more  dense  in  the  lat- 
ter season. 

Caroline.  But,  Mrs.  B.,  if  you  compare  the  weight 
of  equal  quantities  of  different  bodies,  they  will  all  be 
alike.  You  know  the  old  saying,  that  a  pound  of  fea- 
thers is  as  heavy  as  a  pou  nd  of  lead. 

Mrs.  B.  When  therefore  we  compare  the  weight  of 
different  kinds  of  bodies,  it  would  be  absurd  to  take 
quantities  of  equal  iveight,  we  must  take  quantities  of 
equal  bulk  ;  pints  or  quarts,  not  ounces  or  pounds. 

Caroline.  Very  true  ;  I  perplexed  myself  by  thinking 
that  quantity  referred  to  weight,  rather  than  to  measure. 
It  is  true,  it  would  be  as  absurd  to  compare  bodies  of 
the  same  size  in  order  to  ascertain  which  was  largest, 
as  to  compare  bodies  of  the  same  weight  in  order  to 
discover  which  was  heaviest. 

Mrs.  B.  In  estimating  the  specific  gravity  of  bodies, 
therefore,  we  must  compare  equal  bulks,  and  we  shall 
find  that  their  specific  gravity  will  be  proportional  to 
their  weights.  The  body  which  has  been  adopted  as  a 
standard  of  reference  is  distilled  water. 

Emily.  I  am  surprised  that  a  fluid  should  have  been 
chosen  for  this  purpose,  as  it  must  necessarily  be  con- 
tained in  some  vessel,  and  the  ay  eight  of  the  vessel  will 
require  to  be  deducted. 

Mrs.  B.  In  order  to  learn  the  specific  gravity  of  a 
solid  body,  it  is  not  necessary  to  put  a  certaiti  measure 


^ 


MECHANICAL  PROPERTIES  OF  FLUIDS.        187 

of  it  in  one  scale,  and  an  equal  measure  of  water  into  the 
other  scale  :  but  simply  to  wei2;h  the  body  under  trial 
in  water.  If  you  weii^h  a  piece  of  gold  in  a  glass  of 
water,  will  not  the  gold  displace  just  as  much  water,  as 
is  equal  to  its  own  bulk  ? 

Caroline,  Certainly,  where  one  body  is,  another 
cannot  be  at  the  same  time  ;  so  that  a  sutiicient  quantity 
of  water  must  be  -removed,  in  order  to  make  way  for 
the  gold. 

Mrs,  B,  Yes,  a  cubic  inch  of  water  to  make  room 
for  a  cubicinch  of  gold  ;  reaiember  that  the  bulk  alone 
is  to  be  considered,  the  weight  has  nothing  to  do  with 
the  quantity  of  water  displaced,  for  an  inch  of  gold 
does  not  occupy  more  space,  and  therefore  will  not  dis- 
place more  water  than  an  inch  of  ivory,  or  any  other 
substance,  that  will  sink  in  water. 

Well,  you  will  perhaps  be  surprised  to  hear  that  the 
gold  will  weigh  less  in  water,  than  it  did  out  of  it. 

Emily,     And  for  what  reason  } 

Mrs,  B,  On  account  of  the  upward  pressure  of  the 
particles  of  water,  which  in  some  measure  supports  the 
gold,  and  by  so  doing,  diminishes  its  weight.  If  the 
body  immersed  in  water  was  of  the  same  weight  as  that 
fluid,  it  would  be  wholly  supported  by  it,  just  as  the 
water  which  it  displaces  was  supported  previous  to  its 
making  way  for  the  solid  body.  If  the  body  is  heavier 
titan  the  water,  it  cannot  be  wholly  supported  by  it; 
but  the  water  will  offer  some  resistance  to  its  descent. 

Caroline.  And  the  resistance  which  water  offers  to 
the  descent  of  heavy  bodies  immersed  in  it,  (since  it 
proceeds  from  the  upward  pressure  of  the  particles  of 
the  fluid,)  must  in  all  cases,  1  suppose,  be  the  same  ? 


188         MECHANICAL  PROPERTIES  OF  FLUIDS. 

Mrs,  B,  Yes ;  the  resistance  of  the  fluid  is  pro- 
portioned to  the  bulk,  and  not  to  the  weight  of  the  body 
immersed  in  it;  all  bodies  of  the  same  size,  therefore, 
lose  the  same  quantity  of  their  weight  in  water.  Can 
you  form  any  idea  what  this  loss  will  be  ? 

Emily,  I  should  think  it  would  be  equal  to  the 
weight  of  the  water  displaced  ;  for,  since  that  portion 
of  the  water  was  supported  before  the  immersion  of  the 
solid  body,  an  equal  weight  of  the  solid  body  will  be 
supported. 

Mrs.  B,  You  are  perfectly  right :  a  body  weighed  in 
water  loses  just  as  much  of  its  weight,  as  is  equal  to  that 
of  the  water  it  displaces ;  so  that  if  you  were  to  put  the 
water  displaced  into  the  scale  to  which  the  body  is  sus- 
pended, it  would  restore  the  balance. 

You  must  observe,  that  when  you  weigh  a  body  in 
water,  in  order  to  ascertain  its  specific  gravity,  you  must 
not  sink  the  bason  of  the  balance  in  the  water ;  but 
either  suspend  the  body  to  a  hook  at  the  bottom  of  the 
bason,  or  else  take  oft' the  basin,  and  suspend  it  to  the 
arm  of  the  balance,  (fig.  7.)  Now  suppose  that  a  cu- 
bic inch  of  gold  weighed  19  ounces  out  of  water,  and 
lost  one  ounce  of  its  weight  by  being  weighed  in  wa- 
ter, w^iat  would  be  its  specific  gravity  ? 

Caroline,  The  cubic  inch  of  water  it  displaced  must 
weigh  that  one  ounce ;  and  as  a  cubic  inch  of  gold 
weighs  19  ounces,  gold  is  19  times  as  heavy  as  water. 

Emily,  I  recollect  having  seen  a  table  of  the  com- 
parative weights  of  bodies,  in  which  gold  appeared  to  me 
to  be  estimated  at  19  thousand  times  the  weight  of 
water. 

Mrs.  B,    You  misunderstood  the  meaning  of  the 


Mt:CIlANICAL  PROrr/RTiES  OF  FLUIDS.  189 

table.  In  the  estimation  you  allude  to,  the  weight  of 
water  was  reckonetl  at  1000.  Vou  must  observe,  that 
the  weii^ht  of  a  substance  when  not  compared  to  that 
of  any  other,  is  perfectly  arbitrary ;  and  when  water 
is  adopted  as  a  standard,  we  may  denominate  its  weiii;ht 
by  any  number  we  please;  but  then  the  weight  of  all 
bodies  tried  by  this  standard  must  be  sii^nified  by  pit- 
portional  numbers. 

Caroline.  We  may  call  the  weiii^ht  of  water,  for  ex- 
ample, one,  and  then  that  of  gold  wouhf  be  nineteen  ; 
or  if  we  choose  to  call  the  weight  of  water  1000,  that  of 
gold  would  be  19,000.  In  short,  the  specific  gravity 
me  ins  how  much  more  a  body  weighs  than  an  equal 
bulk  of  water. 

Mrs,  B,  It  is  rather  the  weight  of  a  body  compared 
with  that  of  water ;  for  the  specific  gravity  of  many 
substances  is  less  than  that  of  water. 

Caroline,  Then  you  cannot  ascertain  the  specific 
gravity  of  such  substances  in  the  same  manner  as  that  of 
gold  ;  for  a  body  that  is  lighter  than  water  will  float  on 
its  surface  without  displacing  any  water. 

Mrs.  /?.  If  a  body  were  absolutely  light,  it  is  true 
that  it  would  not  displace  a  drop  of  water,  but  the  bo- 
dies we  are  treating  of  have  all  some  weight,  however 
small;  and  will  therefore,  displace  some  quantity  of 
water.  *  If  the  body  be  lighter  than  water,  it  will  not 
sink  to  a  level  with  the  surface  of  the  water,  and  there- 
fore it  will  not  displace  so  much  water  as  is  equal  to 
its  bulk  ;  but  it  will  displace  as  much  as  is  equal  to  its 
weight.  A  ship,  you  must  have  observed,  sinks  to  some 
depth  in  water,  and  the  heavier  it  is  laden  tlie  deeper 


19a      MECHANICAL  PROPERTIES  OF  FLUIDS. 

it  sinks,  as  it  always  displaces  a  quantity  of  water 
equal  to  its  weight* 

Caroline.  But  you  said  just  now,  that  in  the  im- 
mersion of  gold,  the  bulk,  and  not  the  weight  of  body, 
was  to  be  considered. 

Mrs.  B.  That  is  the  case  with  all  substances  which 
are  heavier  than  water;  but  since  those  which  are  light- 
er do  not  displace  so  much  as  their  own  bulk,  the  quan^ 
tity  they  displace  is  not  a  test  of  their  specific  gravity. 

In  order  to  obtain  the  specific  gravity  of  a  body  which 
is  lighter. than  water,  you  must  attach  to  it  a  heavy  one, 
whose  specific  gravity  Is  known,  and  immerse  them  to- 
gether; the  specific  gravity  of  the  lighter  body  may 
then  be  easily  calculated, 

Emily.  But  are  there  not  some  bodies  which  have 
exactly  the  same  specific  gravity  as  water  ? 

Mrs.  B.  Undoubtedly ;  and  such  bodies  will  re- 
main at  rest  in  whatever  situation  they  are  placed  in 
water.  Here  is  a  piece  of  wood  which,  by  being  im- 
pregnated with  a  little  sand,  is  rendered  precisely  of 
the  weight  of  an  equal  bulk  of  water;  in  whatever 
part  of  this  vessel  of  water  you  place  it,  you  will  find 
that  it  will  remain  stationary. 

Caroline.  I  shall  first  put  it  at  the  bottom;  from 
thence,  ©f  course,  it  cannot  rise,  because  it  is  not  light- 
er than  water.  Now  I  shall  place  it  in  the  middle  of 
the  vessel ;  it  neither  rises  nor  sinks,  because  it  is  nei- 
ther lighter  nor  heavier  than  the  water.  Now  I  will 
lay  it  on  the  surface  of  the  water  ;  but  there  it  sinks  a 
little — what  is  the  reason  of  that,  Mrs.  B.  ? 

Mrs.  B.  Since  it  is  not  lighter  than  the  water,  it 
cannot  float  upon  its  surface;  since  it  is  not  heavier 


MECHANICAL  PROPERTIES  OF  FLUIDS.         191 

than  water,  it  cannot  sink  below  its  surface:  it  will 
sink  therefore,  only  till  the  upper  surface  of  both  bo- 
dies are  on  a  level,  so  (hat  the  piece  of  wood  is  just  co- 
vered with  water.  If  you  poured  a  few  drops  of  water 
into  the  vessel,  (so  gently  as  not  to  increase  their  mo- 
mentum by  giving  them  velocity)  they  would  mix  with 
the  water  at  the  surface,  and  not  sink  lower. 

Caroline,  This  must,  no  doubt,  be  the  reason  why 
in  drawing  up  a  bucket  of  water  out  of  a  well,  the 
bucket  feels  so  much  heavier  when  it  rises  above  the 
surface  of  the  water  in  the  well ;  for  whiist  you  raise  it 
in  the  water,  the  water  within  the  bucket  being  of  the 
same  specific  gravity  as  the  water  on  the  outside,  will 
be  wholly  supported  by  the  upward  pressure  of  the  wa- 
ter beneath  the  bucket,  and  consequently  very  little 
force  will  be  required  to  raise  it ;  but  as  soon  as  the 
bucket  rises  to  the  surface  of  the  well  you  immediately 
perceive  the  increase  of  weight. 

Emily.  And  how  do  you  ascertain  the  specific  gra- 
vity of  fluids  ? 

Mrs,  B,  By  means  of  an  instrument  called  an  hy- 
drometer, which  I  will  show  you.  It  consists  of  a  thin 
glass  ball  A,  (fig.  8,  plate  XIII.)  with  a  graduated  tube 
B,  and  the  specific  gravity  of  the  liquid  is  estimated 
1)\  the  depth  to  which  the  instrument  sinl^s  in  it.  There 
is  a  smaller  ball,  C,  attached  to  the  instrument  below, 
which  contains  a  little  mercury ;  but  this  is  merely  for 
the  purpose  of  equipoising  the  instrument,  that  it  may 
remain  upright  in  the  liquid  under  trial. 

I  must  now  take  leave  of  you  ;  but  there  remain  yet 
many  observations  to  be  made  on  fluids ;  we  shall, 
therefore,  resume  this  subject  at  our  next  int&rview. 


CONVERSATION  XI. 


OF  SPRINGS,  FOUNTAINS,  &c. 

OF  THE  ASCEXT  OF  VAPOUR  AND  THE  FORMATION  OF  CLOUDS,——' 
OF  TrtE  FOHMATION  AND  FALL  OF  RAIN,  &C. OF  THE  FORM- 
ATION   OF    SPRINGS. — OP    RIVERS    AND    LAKES. OF     FOUNTAINS. 


t^AROLINE. 

There  is  a  question  I  am  very  desirous  of  asking;  you 
respecting;  fluids,  Mrs,  B.,  which  has  often  perplexed 
me.  What  is  the  reason  that  the  great  quantity  of 
rain  which  falls  upon  the  earth  and  sinks  into  it,  does 
not,  in  the  course  of  time,  injure  its  solidity  ?  The  sun 
and  the  wind,  I  know,  dry  the  surface,  but  they  have 
no  effect  on  the  interior  parts,  where  there  must  be  a 
prodigious  accumulation  of  moisture. 

Mrs.  B.  Do  you  not  know  that,  in  the  course  of  time 
all  the  water  which  sinks  into  the  ground  rises  out  of  it 
again  ?  It  is  the  same  water  which  successively  forms 
Seas,  rivers,  springs,  clouds,  rain^  and  sometimes  hail, 
suow,  and  ice.     If  you  will  cake  the  trouble  of  foUow- 

R 


194  OF  SPRINGS,  FOUNTAINS,   &c. 

ing  it  through  these  various  changes,  you  will  under- 
stand why  the  earth  is  not  vet  drowned  by  the  quantity 
of  water  which  has  fallen  upon  it  since  its  creation ; 
and  you  will  even  be  convinced,  that  it  does  not  con- 
tain a  single  drop  more  water  now,  than  it  did  at  that 
period.  ^ 

Let  us  consider  how  the  clouds  were  originally  form- 
ed. When  the  first  rays  of  the  sun  wanned  the  surface 
of  the  earth,  the  heat,  by  separating  the  particles  of 
water,  rendered  them  lighter  than  the  air.  This,  you 
know,  is  the  case  with  steam  or  vapour.  What  then 
ensues  ? 

Caroline.  When  lighter  than  air  it  will  naturally 
rise  ;  and  now  I  recollect  your  telling  us  in  a  preceding 
lesson,  that  the  heat  of  the  sun  transformed  the  parti- 
cles of  water  into  vapour,  in  consequence  of  which  it 
Ascended  into  the  atmosphere,  where  it  formed  clouds. 
Mrs,  B.  W^e  have  then  already  followed  water 
through  two  of  its  transformations  ;  fiom  water  it  be^ 
comes  vapour,  and  from  vapour  clouds. 

Emily.  But  since  this  watery  vapour  is  lighter  than 
the  air,  why  does  it  not  continue  to  rise  ;  and  why 
does  it  unite  again  to  form  clouds. 

Mrs.  B.  Because  the  atmosphere  diminishes  in  den- 
sity, as  it  is  more  distant  from  the  earth.  The  vapour 
iierefore  which  the  sun  causes  to  exhale,  not  only  from 
seas,  rivers,  and  lakes,  but  likewise  from  the  moisture 
on  the  land,  rises  till  it  reaches  a. region  of  air  of  its 
own  specific  gravity ;  and  there^  you  know,  it  will  re- 
main stationary.  By  the  frequent  accession  of  fresh 
yapour  it  gradually  Accumulates,  so  as  to  form  tho^e 


OIP  SPRINGS,  FOUNTAINS,  &c.  11|6 

larsce  bodies  of  vapour,  which  we  call  clouds;  and 
these,  at  length,  becoming  too  heavy  for  the  air  to  sup- 
port, they  fall  to  the  ground. 

Caroline.  They  do  fall  to  the  ground,  certainly, 
when  it  rains  ;  but,  according  to  your  theory,  I  should 
have  imagined,  that  when  the  clouds  became  too  heavy 
for  the  region  of  air  in  which  they  were  situated  to  sup- 
port them,  they  would  descend  till  they  reached  a  stra- 
tu'^i  of  air   of  t'l-ir  o\^  n  v  ^'*  '  ^,  and   not   f^U    to   t' e 


tfi*j.    Ij,  11     VuU    c.-ciUiJfie    liiC    llUlLliiCl    All      VviliUil      tiiC 

clouds  descend,  it  will  obviate  this  objection.  In  falling, 
several  of  the  watery  particles  come  within  the  sphere  of 
each  other's  attraction,  and  unite  in  the  form  of  a  drop 
ef  water.  The  vapour,  thus  transformed  into  a  shower 
is  heavier  than  any  part  of  th«  atmosphere,  and  conse^ 
quently  descends  to  the  earth. 

Caroline.     How  wonderfully  curious  ! 

Mrs.  B.  It  is  impossible  to  consider  any  part  of  na^ 
ture  attentively  without  being  struck  with  admiration  at 
the  wisdom  it  displays ;  and  I  hope  you  will  never  con- 
template these  wonders  without  feeling  your  heart  glow 
with  admiration  and  gratitude  towards  their  bounteous 
Author.  Observe,  that  if  the  waters  were  never  drawn 
out  of  the  earth,  all  vegetation  would  be  destroyed  by 
the  excess  of  moisture  ;  if,  on  the  other  hand,  the  plants 
were  not  nourished  and  refreshed  by  occasional  showers,, 
the  drought  would  be  equally  fatal  to  them.  If  the 
clouds  constantly  remain  in  a  state  of  vapour,  they 
might,  as  you  remarked,  descend  into  a  heavier  stratum 


i§6  OP  SPRINGS,  FOUNTAINS,  &c. 

of  the  atmosphere,  but  could  never  fall  to  the  ground  ; 
or  were  the  power  of  attraction  more  than  sufficient  to 
convert  the  vapour  into  drops,  it  would  transform  the 
cloud  into  a  mass  of  water,  which,  instead  of  nourishing, 
"Would  destroy  the  produce  of  the  earth. 

Water  then  ascends  in  the  foim  of  vapour,  and  de- 
scends in  that  of  rain,  snow,  or  hail,  all  of  which  ulti- 
mately become  water.  Some  of  this  falls  into  the  various 
bodies  of  water  on  the  surface  of  the  globe,  the  re- 
natinder  upon  the  land.  Of  the  latter,  part  re-ascends 
in  the  form  of  vapour,  part  is  absorbed  bj  the  roots  of 
vegetables,  and  part  descends  into  the  bowels  of  the 
earth,  where  it  forms  springs. 

Emily,     Is  rain  and  spring-water  then  the  same  ? 

Mm.  B,  Yes,  originally!  The  only  difference  be-, 
tween  rain  and  spring  water,  consists  in  the  foreign  par- 
ticles which  the  latter  meets  with  and  dissolves  in  its- 
passage  through  the  various  sods  it  traverses. 

Caroline,  Yet  spring  water  is  more  pleasant  to  the 
taste,  appears  more  transparent,  and,  I  should  have  sup- 
posed, would  have  been  more  pure  than  rain  water. 

Mrs,  Bi  No ;  excepting  distilled  water,  rain  water 
is  the  most  pure  we  can  obtain  ;  and  it  is  its  purity 
which  renders  it  insipid,  whilst  the  various  salts  and 
different  ingredients,  dissolved  in  spring  water,  give  it 
a  species  of  flavour,  without  in  any  degree  affecting  its 
transparency ;  and  the  filtration  it  undergoes  through 
gravel  and  sand  in  the  bowels  of  the  earth,  cleanses  it 
from  all  foreign  matter  which  it  has  not  the  power  of 
dissolving. 

When  rain  falls  on  the  surface  of  the  earth,  it  con- 
tinues making  its  way  downwards  through  the  pores  and 


OF  SPRINGS,  FOUNTAINS,  &c.  197 

crevices  in  the  ground.  When  several  drops  meet  in 
their  subterraneous  passage,  they  unite  and  form  a  lit- 
tle rivulet :  this,  in  its  progress,  meets  with  other  rivulets 
of  a  similar  description,  and  they  pursue  their  course 
together  in  the  bowels  of  the  earth,  till  they  are  stopped 
by  some  substance  which  Ihey  cannot  penetrate. 

Caroline.  '  But  you  said  that  water  could  penetrate 
even  the  pores  of  gold,  and  they  cannot  meet  with  a 
substance  more  dense  ? 

Mrs,  B,  But  water  penetrates  the  pores  of  gold 
only  when  under  a  strong  compressive  force,  as  in  the 
Florentine  experiment ;  now  in  its  passage  towards  the 
centre  of  the  earth,  it  is  acted  upon  by  no  other  power 
than  gravity,  which  is  not  sufficient  to  make  it  force  its 
way  even  through  a  stratum  of  clay.  This  species  of 
earth,  though  not  remarkably  dense,  being  of  great  te« 
naci ty,  will  not  admit  the  particles  of  water  to  pass. 
When  water  encounters  any  substance  of  this  nature 
therefore,  its  progress  is  stopped,  and  the  pressure  of 
the  accumulating  waters  forms  a  bed,  or  reservoir.  This 
will  be  more  clearly  explained  by  fig.  9.  plate  XIII. 
which  represents  a  section,  or  the  interior  of  a  hill  or 
mountain.  A,  is  a  body  of  w^ater  such  as  I  have  de- 
scribed, which,  when  filled  up  as  high  as  B,  (by  the  con- 
tinual accession  of  witer  it  receives  from  the  ducts  or 
rivulets  ft,  a,  «,a,)  finds  a  passage  out  of  the  cavity,  and, 
impelled  by  gravity,  it  runs  on,  till  it  makes  its  way 
out  of  the  ground  at  the  side  of  the  hill,  and  there  forms 
a  spring  C. 

Caroline.     Gravity  impels  downwards  towards  the 
centre  of  the  earth ;  and  the  spring  in  this  figure  runs 
in  an  horizontal  direction. 
R  2 


198  OP  SPRINGS,  FOUNTAINS,  &c. 

Mrs.  B.  Not  entirely.  There  is  some  declivity 
from  the  reservoir  to  the  spot  where  the  water  issues 
out  of  the  ground ;  and  gravity  you  know  will  bring 
bodies  down  an  inclined  plane,  as  well  as  in  a  perpen- 
dicular direction. 

Caroline.  But  though  the  spring  may  descend,  on 
first  issuing,  it  must  afterwards  rise  to  reach  the  surface 
of  the  earth  ;  and  that  is  in  direct  opposition  to  gravity. 

•Mrs,  B,  A  spring  can  never  rise  above  the  level  of 
the  reservoir  whence  it  issues  ;  it  must,  therefore,  find 
a  passage  to  some  part  of  the  surface  of  the  earth  that  is 
lower  or  nearer  the  centre  than  the  reservoir.  It  is 
true  that,  in  this  figure,  the  spring  rises  in  its  passage 
from  B  to  C  occasionally ;  but  this,  I  think,  with  a  lit- 
tle reflection,  you  will  be  able  to  account  for. 

Emily.  Oh,  yes  ;  it  is  owing  to  the  pressure  of  fluids 
upwards,  and  the  water  rises  in  the  duct  upon  the  same 
principle  as  it  rises  in  the  spout  of  a  tea-pot;  that  is  to 
say,  in  order  to  preserve  an  equilibrium  with  the  water 
in  the  reservoir.  Now  I  think  I  understand  the  nature 
of  springs:  the  water  will  flow  through  a  duct,  whetiier 
ascending  or  descending,  provided  it  never  rises  higher 
than  the  reservoir. 

Mrs,  B.  Water  may  thus  be  conveyed  to  every  part 
of  a  town,  and  to  the  upper  part  of  the  houses,  if  it  is 
originally  brought  from  a  height  superior  to  any  to 
which  it  is  conveyed.  Ha\e  you  never  observed,  when 
the  pavement  of  the  streets  have  been  mending,  the 
pipes  which  serve  as  ducts  for  the  conveyance  of  the 
water  through  the  town  ? 

Emily,  Yes,  frequently;  and  I  have  remarked  that 
when  any  of  these  pipes  have  been  opened,  the  water 


Put.   'J. 


Fi^.2. 


Fu].4. 


Fig.  5 


Fiq.H  I        ! 


'^jm-     ■ 


Fu).6. 


,,, E- 


iuh.  hv  J.  \'.  Uuniplti.ys  diilu.i:- 


OF  SPRINGS,  FOUNTAINS,  &c.  199 

ruslies  upwards  from  them  with  great  velocity,  which  I 
suppose,  proceeds  from  the  pressure  of  the  water  in  the 
reservoir,  which  forces  it  out. 

Caroline.  1  recollect  having;  once  seen  a  very  curi- 
ous glass,  called  Tantalus's  cup  ;  it  consists  of  a  goblet, 
containing  a  small  figure  of  a  man,  and  whatever  quan- 
tity of  water  you  pour  into  the  goblet,  it  never  rises 
higher  than  the  breast  of  the  figure.  Do  you  know  how 
that  is  contrived  ? 

Mrs,  B.  It  is  by  means  of  a  syphon,  or  bent  tube, 
which  is  concealed  in  the  body  of  the  figure.  It  rises 
through  one  of  the  legs  as  high  as  the  breast,  and  there 
taming  descends  through  the  other  leg,  and  from  thence 
through  the  foot  of  the  goblet,  where  the  water  runs  out. 
(fig.  I.  plate  XIV.)  When  you  pour  water  into  the  glass 
A,  it  must  rise  in  the  syphon  B,  in  proportion  as  it  rises 
in  the  glass  ;  and  when  the  glass  is  filled  to  a  level  with 
the  upper  part  of  the  syphon,  the  water  will  run  out 
through  the  other  leg  of  the  figure,  and  will  continue 
running  out,  as  fast  as  you  pour  it  in ;  therefore  the 
glass  can  never  fill  any  higher. 

Emily,  I  think  the  new  well  that  has  been  made  at 
our  country-house,  must  be  of  that  nature.  We  had  a 
great  scarcity  of  water,  and  my  father  has  been  at  con- 
siderable expense  to  dig  a  well ;  after  penetrating  to  a 
great  depth  before  water  could  be  found,  a  spring  was 
at  length  discovered,  but  the  water  rose  only  a  few  feet 
above  the  bottom  of  the  well ;  and  sometimes  it  is 
quite  dry.  . 

Mrs,  B,  This  has,  however,  no  analogy  to  Tanta- 
lus's cup,  but  is  owing  to  the  very  elevated  situation  of 
your  country-house. 


20^  OF  SPRINGS,  FOItNTAINS,  8cc. 

Emily,  I  believe  I  guess  the  reason.  There  cannot 
be  a  reservoir  of  water  near  the  summit  of  a  hill ;  as  in 
ouch  a  situation,  there  will  not  be  a  sufficient  number 
of  rivulets  formed  to  supply  one ;  and  without  a  reser- 
voir, there  can  be  no  spring.  In  such  situations,  there- 
fore, it  is  necessary  to  dig  very  deep,  in  order  to  meet 
with  a  spring;  and  when  we  give  it  vent,  it  can  rise 
only  as  high  as  the  reservoir  from  whence  it  flows, 
which  will  be  but  little,  as  the  reservoir  must  be  situated 
at  some  considerable  depth  below  the  summit  ot  the  hill. 

Caroline*  Your  explanation  appears  very  clear  and 
satisfactory ;  but  I  can  contradict  it  from  experience. 
At  the  very  top  of  a  hill,  near  our  country-house,  there 
is  a  large  pond,  and,  according  to  your  theory,  it  would 
be  impossible  there  should  be  springs  in  such  a  situation 
to  supply  it  with  water.  Then  you  know  that  1  have 
crossed  the  Alps,  and  I  can  assure  you,  that  there  is  a 
fine  lake  on  the  summit  of  Mount  Cenis,  the  highest 
mountain  we  passed  over. 

Mrs.  B,  Were  there  a  lake  on  the  summit  of  Mount 
Blanc,  which  is  the  highest  of  the  Alps,  it  would  indeed 
be  wonderful.  But  that  on  Mount  Cenis,  is  not  at  all 
contradictory  to  our  theory  of  springs;  for  this  moun- 
tain is  surrounded  by  others,  much  more  elevated,  and 
the  springs  which  feed  the  lake  must  descend  from  re- 
servoirs of  water  formed  in  those  mountains.  This 
must  also  be  the  case  with  the  pond  on  the  top  of  the 
hill:  there  is  doubtless  some  more  considerable  hill  in 
the  neighbourhood,  which  supplies  it  with  water. 

Emily.  I  comprehend  perfectly,  why  the  water  in 
our  well  never  rises  high  :  but  I  do  not  understand  why 
it  should  occasionally  be  dry. 


OF  SP1?TNG5,  FOUNTAINS,  &c.  QOl 

Mrs,  B,  Because  the  reservoir  from  which  it  flows, 
being;  in  an  elevated  situation,  is  but  scantily  supplied 
with  water;  after  a  long;  dmught,  therefore,  it  may  be 
drained,  and  the  spring  dry,  till  the  reservoir  be  re- 
plenished by  fresh  rains.  It  is  not  uncommon  to  see 
springs  flow  with  ^reat  violence  in  wet  weather,  and  at 
other  times  be  perfectly  dry. 

Caroline,  But  there  is  a  spring  in  our  p;rounds 
which  more  frequently  flows  in  dry  than  in  wet  wea- 
ther :  how  is  that  to  b^^  accounted  for  ? 

Mrs.  B.  The  spring  probably  comes  from  a  reservoir 
at  a  great  distance,  and  situated  very  deep  in  the 
ground  :  it  is,  therefore,  some  length  of  time  before  the 
rain  reaches  the  reservoir,  and  another  considerable  por- 
tion must  elapse,  whilst  the  water  is  making  its  way 
from  the  reservoir  to  the  surface  of  the  earth  ;  so  that 
the  dry  weather  may  probably  have  succeeded  the  rain& 
before  the  spring  begins  to  flow,  and  the  reservoir  may 
be  exhausted  by  the  time  the  wet  weather  sets  in 
again. 

Caroline.  I  doubt  not  but  this  is  the  case,  as  the 
spring  is  in  a  very  low  situation,  therefore  the  reser- 
voir may  be  at  a  g  eat  distance  from  it. 

Mrs.  B.  Springs  which  do  not  constantly  flow,  are 
called  intermitting,  and  are  occasioned  by  the  reser- 
voir being  imperfectly  supplied.  Independently  of  the 
situation,  this  is  always  the  case  when  the  duct  or 
ducts  which  convey  the  water  into  the  reservoir  are 
smaller  than  those  which  carry  it  off. 

Caroline.  If  it  runs  out  faster  than  it  runs  in,  it 
will  of  course  sometimes  be  empty.  And  do  not  rivers 
also  derive  their  source  from  springs  ? 


2QS  OF  SPRINGS,  FOUNTAINS,  &c. 

•^rs.  B.  Yes,  they  generally  take  their  fource  m 
mountainous  countries,  where  springs  are  most  abun- 
dant. 

Caroline,  I  understood  you  that  springs  were  more 
rare  in  elevated  situations. 

J\Irs,  B.  You  do  not  consider  that  mountainous  coun- 
tries abound  equally  with  high  and  low  situations.  Re- 
servoirs of  water,  which  are  formed  in  the  bosom  of 
moiintnins,  s»^pnorally  find  a  vent  either  on  th«^ii-  dec!l»' 

di•.^ov('i•^si  by  *<il^r:^in^  :i  wv'A,  When  a  spring  once 
issues  at  the  surface  of  the  earth  it  continues  its  course 
externally,  seeking  always  a  lower  ground,  for  it  cau 
nojonger  rise. 

Emily,  Then  what  is  the  consequence,  if  the  spring, 
or  I  should  now  rather  call  it  a  rivulet,  runs  into  a  si- 
tuation, which  is  surrounded  by  higher  ground. 

Mrs.  B.  Its  course  is  stopped,  the  water  accumu- 
lates, and  it  forms  a  pool,  pond,  or  lake,  according  to 
the  dimensions  of  the  body  of  water.  The  lake  of  Ge- 
neva, in  all  probability,  owes  its  origin  to  the  Rhone, 
which  passes  through  it :  if,  when  this  river  first  en- 
tered the  valley,  which  now  forms  the  bed  of  the  Lake^ 
it  found  itself  surrounded  by  higher  grounds,  its  wa- 
ters would  there  accumulate,  till  they  rose  to  a  level 
with  that  part  of  the  valley,  where  the  Rhone  now  con- 
tinues its  course  beyond  the  Lake,  and  from  whence  it 
flows  through  valleys,  occasionally  formfng  other  small 
lakes  till  it  reaches  the  sea. 


0P  SPRINGS,  FOUNTAINS,  Sec.  2^ 

Emily.  And  are  not  fountains  of  the  nature  of 
springs  ? 

Mrs,  B,  Exactly.  A  fountain  is  conducted  per- 
pendicularly upwards,  by  the  spout  or  adjutage  A, 
through  which  it  flows  ;  and  it  will  rise  nearly  as  high 
as  the  reservoir  B,  from  whence  it  proceeds.  (Plate 
XIV.  figure  2.) 

Caroline.     Why  not  quite  as  high  ? 
Mrs.  B.     Because  it  meets  with  resistance  from  the 
air  in  its  accent;  and  its  motion  is  impeded  by  friction 
agaiiist  the  spout,  where  it  rushes  out. 

Emily,  But  if  the  tube  through  which  the  water 
rises  be  smooth,  can  there  be  any  friction  ?  especially 
with  a  fluid,  whose  particles  yield  to  the  slightest  im- 
pression. 

Mrs.  B.  Friction,  (as  we  observed  in  a  former  les- 
son,) may  be  diminished  by  polishing,  but  can  never  be 
entirely  destroyed  ;  and  though  fluids  are  less  suscepti- 
ble of  friction  than  solid  bodies,  they  are  still  affected  by 
it  Another  reason  why  a  fountain  will  not  rise  so  high 
as  its  reservoir,  is,  that  as  all  the  particles  of  water 
spout  from  the  tube  with  an  equal  velocity,  and  as  the 
pressure  of  the  air  upon  the  exterior  particles  must  di- 
minish their  velocity,  they  will  in  some  degree  strike 
against  the  under  parts,  and  force  them  sideways, 
spreading  the  column  into  a  head,  and  rendering  it  botk 
wider  and  shorter  than  it  otherwise  would  be. 

At  >ur  next  meeting,  we  shall  examine  the  mechanic 
eal  properties  of  the  air,  which  being  an  elastic  fluick 
differs  in  many  respects  from  liquids. 


CONVERSATION  XII. 


i)N  THE  MECHANICAL  PROPERTIES  OF  AIR. 

OF   THE    SPRIXG    OR    ELASTICITY   OF    THE   AIR. OP     THE    WEIGHT 

OF   THE    AIR. EXPERIMENTS    WITH    THE    AIR   PUMP. OF   THE 

BAROMETER. MODE    OF    WEIGHING    AIR. SPECIFIC  GRAVITY  OP 

AIR. OF    PUMPS. DESCRIPTION     OF   THE    SUCKING   PUMP.— DE-* 

SCRIPTION    OP   THE   FORCING   PUMP. 


Mrs.  B. 

At  our  last  meeting  we  examined  the  properties  of 
fluids  ia  general,  and  more  particularly  of  such  fluids 
as  are  called  liquids. 

There  is  another  class  of  fluids,  distinguished  by  the 
name  of  aeriform  or  elastic  fluids,  the  principal  of  which 
is  the  air  we  breathe,  which  surrounds  the  earth,  and  is 
called  the  atmosphere. 

Emily.  There  are  then  other  kinds  of  air,  besides  the 
atmosphere  ? 

Mrs.  B.  Yes  ;  a  great  variety ;  but  they  differ  only 
in  their  chemical,  and  not  in  their  mechanical  proper- 


206  MECriANICAL  PROPERTIES  OF  AIR. 

ties ;  and  as  it  is  the  latter  we  are  to  examine,  we  shall 
notaipresent  inquire  into  their  cojh position,  but  confine 
our  attention  to  the  mechanical  properties  of  elastic 
fluids  in  general. 

Caroline.     And  from  whence  arises  this  difterence  ? 

Mrs,  B.  There  is  no  attraction  of  cohesion  between 
the  particles  of  elastic  fluids;  so, that  the  expansive 
power  of  heat  has  no  adversary  to  contend  with  but  gra- 
vity ;  any  increase  of  temperature,  therefore,  expands 
elastic  fluids  prodigiously,  and  a  diminution  proportion- 
ally condenses  them. 

The  most  essential  point  in  which  air  differs  from 
other  fluids,  is  by  its  spring  or  elasticity  ;  that  is  to  say, 
its  power  of  increasing  or  diminishing  in  bulk,  accor- 
ding as  it  is  more  or  le&s  compressed  :  a  power  of 
which  I  have  informed  you  liquids  are  almost  wholly 
deprived. 

Emily,  I  think  I  understand  the  elasticity  of  the  air 
very  well  from  what  you  formerh  said  of  it ;  (see  p.  42.) 
but  what  pel  plexes  me  is,  its  having  gravity ;  if  it  is 
heavy  and  we  are  w«urrounded  by  it,  why  do  we  not  feel 
its  weight? 

Caroline,  It  must  be  impossible  to  be  sensible  of  the 
weight  of  such  infinitely  small  particles,  as  those  of 
which  the  air  is  composed  :  particles  which  are  too 
small  to  be  seen,  must  be  too  light  to  be  felt. 

Mrs.  B,  You  are  mistaken,  my  dear  ;  the  air  is 
much  heavier  than  you  imagine  ;  it  is  true,  that  the  par- 
ticles which  compose  it  are  small ;  but  then,  reflect  on 
their  quantity :  the  atmosphere  extends  to  about  the 
distance  of  45  miles  from  the  earth,  and  its  gravity  is 
«uch>  that  a  man  of  middling  stature  is  computed  (wheft 


MECHANICAL  PROPERTIES  OP  AIR.  207 

the  air  is  heaviest)  to  sustain  the  weight  of  about  14 
tons. 

Caroline.  Is  it  possible  !  I  should  have  thought  such 
a  wei2;ht  would  have  crushed  any  one  to  atoms. 

t/l/rs.  B,  That  would,  indeed,  be  the  case,  if  it  were 
not  for  the  equality  of  the  pressure  on  every  part  of  the 
body ;  but  when  thus  diffused,  we  can  bear  even  a 
much  greater  weight,  without  any  considerable  incon- 
venience. In  bathing  we  support  the  weight  and  pres* 
sure  of  the  water,  in  addition  to  that  of  the  atmosphere  ; 
but  because  this  pressure  is  equally  distributed  over 
the  body,  we  are  scarcely  sensible  of  it ;  whilst  if  your 
shoulders,  your  head,  or  any  particular  part  of  your  frame 
were  loaded  with  the  additional  weight  of  a  hundred 
pounds  you  would  soon  sink  under  the  fatigue.  Be- 
sides this  our  bodies  contain  air,  the  spring  of  which 
counterbalances  the  weight  of  the  external  air,  and  ren- 
ders us  less  sensible  of  its  pressure. 

Caroline.  .  But  if  it  were  possible  to  relieve  me  from 
the  weight  of  the  atmosphere,  should  I  not  feel  more 
light  and  agile  ? 

Mrs.  B.  On  the  contrary,  the  air  within  you  meet- 
ing with  no  external  pressure  to  restrain  its  elasticity, 
would  distend  your  body,  and  at  length  bursting  the 
parts  which  confined  it,  put  a  period  to  your  existence. 

Caroline.  This  weight  of  the  atmosphere,  then, 
which  I  was  so  apprehensive  would  crush  me,  is,  in 
reality,  essential  to  my  preservation. 

Emily.  I  once  saw  a  person  cupped,  and  was  told 
that  the  swelling  of  the  part  under  the  cup  was  produ- 
ced by  taking  away  from  that  part  the  pressure  of  the 


208  MECHANICAL  PROPERTIES  OF  AIR. 

atmosphere;  but  I  could  not  understand  how  this  pres- 
sure produced  such  an  effecl. 

Mrs,  B,  The  air  pump  affords  us  the  means  of  ma- 
king a  great  variety  of  interesting  experiments  on  the 
weight  and  pressure  of  the  air  :  some  of  them  you  have 
already  seen.  Do  you  not  recollect,  that  in  a  vacuum 
produced  within  the  air  pump,  substances  of  various 
weights  fell  to  the  bottom  in  the  same  time ;  why  does 
not  this  happen  in  the  atmosphere  ? 

Caroline,  I  remember  you  told  us  it  was  owing  to 
the  resistance  which  light  bodies  meet  with  from  the  air 
during  their  fall. 

Mrs,  B,  Or,  in  other  words,  to  the  support  which 
they  received  from  the  air,  and  which  prolonged  the 
time  of  their  fall.  Now,  if  the  air  were  destitute  of 
weight,  how  could  it  support  other  bodies,  or  retard 
their  fall  ? 

1  shall  now  show  you  some  other  experiments,  which 
illustrate,  in  a  striking  manner,  both  the  weight  and 
elasticity  of  air.  I  shall  tie  a  piece  of  bladder  over 
this  glass  receiver,  v;hich,  you  will  observe,  is  open 
both  at  the  top  as  well  as  below. 

Caroline,     Why  do  you  wet  the  bladder  first  ? 

Mrs,  B,  It  expands  by  wetting,  and  contracts  in 
drying ;  it  is  also  more  feoft  and  pliable  when  wet,  so 
that  I  can  make  it  fit  better,  and  when  dry  it  will  be 
tighter.  We  must  hold  it  to  the  fire  in  order  to  dry  ; 
but  not  too  near  lest  it  should  burst  by  sudden  contrac- 
tion. Let  us  now  fix  it  on  the  air-pump  and  exhaust 
the  air  from  underneath  it-— you  will  not  be  alarmed  if 
you  hear  a  noise  ? 


MECHANICAL  PROPERTIES  OP  AIR.  209 

Emily.  It  was  as  loud  as  the  report  of  a  gun,  and  the 
bladder  is  burst !  Praj  explain  how  the  air  is  concerned 
in  this  experiment.  ♦ 

Mrs,  B.  It  is  the  effect  of  the  weight  of  the  atmos- 
phere on  the  upper  surface  of  the  bladder,  when  I  had 
taken  away  the  air  from  the  under  surface ;  so  that 
there  was  no  longer  any  reaction  to  counterbalance  the 
pressure  of  the  atmosphere  on  the  receiver.  You  ob- 
served how  tlie  bladder  was  pressed  inwards  by  the 
weight  of  the  external  air,  in  proportion  as  I  exhaust* 
ed  the  receiver:  and  before  a  complete  vacuum  was 
formed,  the  bladder  unable  to  sustain  the  violence  of 
the  pressure,  burst  with  the  explosion  you  have  just 
heard. 

I  shall  now  show  you  an  experiment,  which  proves 
the  expansion  of  the  air,  contained  within  a  body  when 
it  is  relieved  from  the  pressure  of  the  external  air. 
You  would  not  imagine  that  there  was  any  air  contained 
within  this  shrivelled  apple,  by  its  appearance;  but 
take  notice  of  it  when  placed  within  a  receiver,  from 
which  1  shall  exhaust  the  air.  \ 

Caroline,  How  strange  ;  it  grows  quite  plump,  and 
looks  like  a  fresh-gathered  apple. 

Mrs,  B.  But  as  soon  as  1  let  the  air  again  into  the 
receiver,  the  apple  you  see  returns  to  its  shrivelled 
state.  When  [  took  away  the  pressure  of  the  atmosphere 
the  air  within  the  apple  expanded  and  swelled  it  out; 
but  the  instant  the  atmospherical  air  was  restored,  the 
expansion  of  the  internal  air  uas  checked  and  repress 
sed,  aad  the  apple  shrunk  to  its  former  dimensions. 

You  may  make  a  similar  experiment  with  this  little 
bladder,  which  you  see  is  perfectly  iiaccid  and  appears 
s2 


210  MECHANICAL  PROPERTIES  OF  AIR. 

to  contain  no  air  :  in  this  state  I  shall  tie  up  the  neck  of 
the  bladder,  so  that  whatever  air  remains  within  it  may 
not  escape,  and  then  place  it  under  the  receiver.  Now 
observe,  as  I  exhaust  the  receiver,  how  the  bladder  dis- 
tends ;  this  proceeds  from  the  great  dilatation  of  the 
small  quantity  of  air  which  was  inclosed  within  the  blad- 
der when  I  tied  it  up  ;  but  as  soon  as  I  let  the  air  into 
the  receiver,  that  which  the  bladder  contains,  conden- 
ses and  shrinks  into  its  small  compass  within  the  folds 
of  the  bladder. 

Emily,  These  experiments  are  extremely  amusing 
and  they  afford  clear  proofs  both  of  the  weight  and  elais- 
ticity  of  the  air ;  but  I  should  like  to  know  exactly  how 
much  the  air  weighs. 

Mr^s,  B,  A  column  ofair  reaching  to  the  top  of  the  at- 
mosphere, and  whose  base  is  a  square  inch,  weighs  15lb 
when  the  air  is  heaviest;  therefore  every  square  inch 
of  our  bodies  sustains  a  weight  of  15lbs. :  and  if  you 
wish  to  know  the  weight  of  tlie  whole  of  the  atmosphere, 
you  must  reckon  how  many  square  inches  there  are  on 
the  surface  of  the  globe, and  multiply  them  by  15. 

Emily,  But  are  there  no  means  of  ascertaining  the 
weight  of  a  small  quantity  ofair  ? 

Mrs.  B,  Nothing  more  easy.  I  shall  exhaust  the  air 
from  this  little  bottle  by  means  of  the  air-pump :  and 
having  emptied  the  bottle  of  air,  or,  in  other  words,  pro- 
duced a  vacuum  within  it  1  secure  it  by  turning  this 
sqrew  adapted  to  its  neck  :  we  may  now  find  the  exact 
weight  of  this  bottle,  by  putting  it  into  one  of  the  scales 
of  a  balance.  It  weighs  you  see  just  two  ounces  ;  but 
when  I  turn  the  screw,  so  as  to  admit  the  air  into  the 
bottle  the  scale  which  contains  it  preponderates. 


MECHANICAL  PROPERTIES  OF  AIR.  211 

Caroline.  No  doubt  the  bottle  filled  with  air,  is 
heavier  than  the  bottle  void  of  air ;  and  the  addi- 
tional weight  required  to  bring  the  scales  again  to  a  bal- 
ance, must  be  exactly  that  of  the  air  which  the  bottle 
now  contains. 

Jl/rs.  B,  That  weight,  you  see,  is  almost  two  grains. 
The  dimensions  of  this  bottle  are  six  cubic  inches.  Six 
cubic  inches  of  air,  therefore,  at  the  temperature  of 
this  room,  weighs  nearly  2  grains. 

Caroline,  Why  do  you  observe  the  temperature  of 
the  room,  in  estimating  the  weight  of  the  air. 

Mrs.  B,  Because  heat  mrifies  air,  and  renders  it 
lighter ;  therefore  the  warmer  the  air  is  which  you 
weigh,  the  lighter  it  will  be. 

If  you  should  now  be  desirous  of  knowing  the  spe- 
cific gravity  of  this  air,  we  need  only  fill  the  same  bot- 
tle with  water,  and  thus  obtain  the  weight  of  an  equal 
quantity  of  water— which  you  see  is  1515  grs. ;  now  by 
comparing  the  weight  of  water  to  that  of  air,  we  find 
it  to  be  in  the  proportion  of  about  800  to  1. 

I  will  show  you  another  instance  of  the  weight  of 
the  atmosphere,  which  I  think  will  please  you :  you 
know  what  a  barometer  is  ? 

Caroline,  It  is  an  instrument  which  indicates  the 
state  of  the  weather,  by  means  of  a  tube  of  quicksilver; 
but  how,  I  cannot  exactly  say. 

Mrs,  B,  It  is  by  showing  the  weight  of  the  atmos- 
phere. The  barometer  is  an  instrument  extremely  sim- 
ple in  its  construction  :  in  order  that  you  may  under- 
stand it,  I  will  show  you  how  it  is  made.  I  first  fill  a 
glass  tube  A  B,  (fig.  3.  plate  XIV.)  about  three  feet  in 
length,  and  open  only  at  one  end,  with  mercury ;  then 


21^  MECHANICAL  PROPERTIES  OP  AIR. 

stopping  the  open  end  with  my  finger.  I  immerse  it  in  a 
cup  C,  containing  a  little  mercury. 

Emily.  Part  of  the  mercury  which  was  in  the  tube,  I 
observe,  runs  clown  into  the  cup ;  but  why  does  not  the 
whole  of  it  subside  in  the  cup,  for  it  is  contrary  to  the 
law  of  the  equilibrium  of  fluids,  that  the  mercury  in  the 
tube  should  not  descend  to  a  level  with  that  in  the  cup. 

Mrs.  B.  The  mercury  that  has  fallen  from  the  tube 
into  the  cup,  has  left  a  vacant  space  in  the  upper  part 
of  the  tube,  to  which  the  air  cannot  gain  access;  this 
space  is  therefore  a  perfect  vacuum  ;  and  consequently 
the  mercury  in  the  tube  is  relieved  from  the  pressure  of 
the  atmosphere,  whilst  that  in  the  cup  remains  exposed 
to  it. 

Caroline.  Oh,  now  I  understand  it;  the  pressure  of 
the  air  on  the  mercury  in  the  cup  forces  it  to  rise  in  the 
tube,  where  it  sustains  no  pressure. 

Emily.  Or  rather  supports  the  mercury  in  the  tube, 
and  prevents  it  from  falling. 

Mrs.  B.  That  comes  to  the  same  thing ;  for  the 
power  that  can  support  mercury  in  a  vacuum,  would 
also  make  it  ascend  when  it  met  with  a  vacuum. 

Thus  you  see,  that  the  equilibrium  of  the  mercury  is 
destroyed  only  to  perscrve  the  general  equilibrium  of 
fluids. 

Caroline.  But  this  simple  apparatus  is,  in  appear- 
ance, very  unlike  a  barometer. 

Mrs.  B,  It  is  all  tnat  is  essential  to  a  barometer. 
The  tube  and  the  cup  or  vase  are  fixed  on  a  board,  for 
the  convenience  of  suspending  it;  the  board  is  gradu- 
ated for  the  purpose  of  ascertaining  the  height  at  which 
the  mercury  stands  in  the  tube ;  and  tiie  small  move* 


I 


MECHANICAL  PROPERTIES  OF  AIR.  21« 

able  metal  plate  serves  to  show  that  height  with  greater 
accuracy. 

Emily.  And  at  what  height  will  the  weight  of  the 
atmosphere  sustain  the  mercury  ? 

Mrs,  B.  About  28  inches,  as  you  will  see  by  this 
barometer ;  but  it  depends  upon  the  weight  of  the  at- 
mosphere, which  varies  much  according  to  the  state  of 
the  weather.  The  greater  the  pressure  of  the  air  on  the 
mercury  in  the  cup,  the  higher  it  will  ascend  in  the 
tube.  Now  can  you  tell  me  whether  the  air  is  heavier 
in  wet  or  dry  weather  ? 

Caroline.  Without  a  moment's  reflection,  the  air 
must  be  heaviest  in  wet  weather.  It  is  so  depressing, 
and  makes  one  feel  so  heavy ;  while  in  fine  weather,  I 
feel  as  light  as  a  feather,  and  as  brisk  as  a  bee. 

Mrs,  B.  Would  it  not  have  been  better  to  have  an- 
swered with  a  moment's  reflection,  Caroline  ?  It  would 
have  convinced  you,  that  the  air  must  be  heaviest  in 
dry  weather,  for  it  is  then,  that  the  mercury  is  found  to 
rise  in  the  tube,  and  consequently  the  mercury  in  the 
cup  must  be  most  pressed  by  the  air :  and  you  know, 
that  we  estimate  the  dryness  and  fairness  of  the  wea- 
ther, by  the  height  of  the  mercury  in  the  barometer. 

Caroline.  Why  then  does  the  air  feel  so  heavy  in 
bad  weather  ? 

Mrs.  B.  Because  it  is  less  salubrious  when  impreg- 
nated with  damp.  The  lungs  under  these  circumstances 
do  not  play  so  freely,  nor  does  the  blood  circulate  so 
well :  thus  obstructions  are  frequently  occasioned  in 
the  smaller  vessels,  from  which  arise  colds,  asthmas, 
agues,  fevers,  &c. 


214  MECHANICAL  PROPERTIES  OP  AIR. 

Emily,  Since  the  atmosphere  diminishes  in  density 
in  the  upper  regions,  is  not  the  air  more  rare  upon  a  hill 
than  in  a  plain  ;  and  does  the  barometer  indicate  this 
difference  ? 

Mrs,  B,  Certainly.  The  hills  in  this  country  are 
not  sufficiently  elevated  to  produce  any  very  consi- 
derable effect  on  the  barometer;  but  this  instrument  is 
so  exact  in  its  indications,  that  it  is  used  for  the  pur- 
pose of  measuring  the  height  of  mountains,  and  of  esti- 
mating the  elevation  of  balloons. 

Emily,  And  is  no  inconvenience  experienced  from 
the  thinness  ol  the  air  in  such  elevated  situations  ? 
.  Mrs,  B,  Oh,  yes  ;  frequently.  It  is  sometimes  op- 
pressive, from  being  insufficient  for  respiration;  and 
the  exp:\nsion  which  takes  place  in  the  more  dense  air 
contained  within  the  Body  is  often  painful :  it  occasions 
distension,  and  sometimes  causes  the  bursting  of  the 
smaller  blood-vessels  in  the  nose  and  ears.  Besides, 
in  such  situations,  you  are  more  exposed  both  to  heat 
and  cold  ;  for  though  the  atmosphere  is  itself  transpa- 
rent, its  lower  regions  abound  with  vapours  and  ex- 
halations from  the  earth,  which  float  in  it,  and  act  in 
sotne  degree  as  a  covering,  which  preserves  us  equally 
from  the  intensity  of  the  sun's  rays,  and  from  the  seve- 
rity of  the  cold. 

Caroline,  Pray,  Mrs.  B.,  is  not  the  thermometer 
constructed  on  the  same  principles  as  the  barometer  ? 

Mrs,  B.  Not  at  all.  The  rise  and  fail  of  the  fluid 
in  the  thenijometer  is  occasioned  by  the  expansive 
power  of  heat,  and  the  condensation  produced  by  cold  ; 
the  air  has  no  a'  cess  to  it.  An  exfslanation  of  it  would, 
thereioiebe  irrelevant  to  Qur  prestni  subject. 


MECHANICAL  PROPERTIES  OF  AIR.  215 

Emily.  I  have  been  reflecting,  that  since  it  is  the 
weight  of  the  atmos|3here  which  supports  the  mercury 
in  the  tube  of  a  barometer,  it  would  support  a  co'umn 
of  any  other  fluid  in  the  same  manner. 

Mrs,  B,  Certainly ;  but  as  mercury  is  heavier  than 
all  other  fluids,  it  will  support  a  hig;her  column  of  any 
other  fluid  ;  for  two  fluids  are  in  equilibrium,  when  their 
heii^ht  varies  inversely  as  their  densities.  We  find  the 
weight  of  the  atmosphere  is  equal  to  sustaining  a  co- 
lumn of  water,  for  instance,  of  no  less  than  32  feet 
above  its  level. 

Caroline.  The  weight  of  the  atmosphere,  is  then,  as 
great  as  that  of  a  body  of  water  the  depth  of  32  feet  ? 

Mrs.  B.  Precisely ;  for  a  column  of  air  of  the  height 
of  the  atmosphere  is  equal  to  a  column  of  water  of  32 
feet,  or  one  of  mercury  of  28  inches. 

The  common  pump  is  constructed  on  this  principle. 
By  the  act  of  pumping,  the  pressure  of  the  atmosphere 
is  taken  off  the  water,  which,  in  consequence,  rises. 

The  body  of  a  pump  consists  of  a  large  tube  or  pipe, 
whose  lower  end  is  immersed  in  the  water  which  it  is 
designed  to  raise.  A  kind  of  stopper,  called  a  piston, 
is  fitted  to  this  tube,  and  is  made  to  slide  up  and  down 
it  by  means  of  a  metallic  rod  fastened  to  the  centre  of 
the  piston. 

Emily.  Is  it  not  similar  to  the  syringe,  or  squirt, 
with  which  you  first  draw  in,  and  then  force  out  water  P 

Mrs.  B.  It  is  ;  but  you  know  that  we  do  not  wish 
to  force  the  water  out  of  the  pump,  at  the  same  end  of 
the  pipe  at  which  we  draw  it  in.  The  intention  of  a 
pump  is  to  raibe  water  from  a  S|)ring  or  well ;  the  pipe 


216  MECHANICAL  PROPERTIES  OP  AIR. 

is,  tijerefore,  placed  perpendicularly  over  the  water 
which  enters  it  at  the  lower  extreiriitj,  and  it  issues  at 
a  horizontal  spout  towards  the  upper  part  of  the  pump. 
The  punop,  therefore,  is  rather  a  more  complicated  piece 
of  machinery  than  the  syringe. 

Its  various  parts  are  delineated  in  this  figure  :  (fig.  4. 
plaie  XIV.)  A  B  is  the  pipe  or  body  of  the  pump,  P 
the  piston,  V  a  valve,  or  little  door  in  the  piston,  which, 
opening  upwards,  admits  the  water  to  rise  through  it, 
but  prevents  its  returning,  and  Y  a  similar  valve  in  the 
body  of  the  pump. 

When  the  pump  is  in  a  state  of  inaction,  the  two 
valves  are  closed  by  their  own  weight ;  but  when,  by 
drawing  down  the  handle  of  the  pump,  the  piston  as- 
cends, it  raises  a  column  of  air  which  rested  upon  it, 
and  produces  a  vacuum  between  the  piston  and  the 
lower  valve  Y,  the  air  beneath  this  valve,  which  is  im- 
mediately over  the  surface  of  the  water,  consequently 
expands,  and  forces  its  way  through  it:  the  water,  thei^ 
relieved  from  the  pressure  of  the  air,  ascends  into  the 
pump.  A  few  strokes  of  the  handle  totally  excludes 
the  air  from  the  body  of  the  pump,  and  fills  it  with  wa- 
ter, which,  having  passed  through  both  the  valves,  runs 
out  at  the  spout. 

Caroline.  1  understand  this  perfectly.  When  the 
piston  is  elevated,  the  air  and  the  water  successively 
rise  in  the  pump;  for  the  same  reason  as  the  mercury 
rises  in  the  barometer. 

Emily.  1  thought  that  water  was  drawn  up  into  a 
pump,  by  suction,  in  the  same  manner  as  water  may  be 
sucked  through  a  straws* 


MBCHANICAL  PROPERTIES  OF  AIR.  217 

Mrs.  B.  It  is  so,  into  the  body  of  the  pump;  for 
the  power  of  suction  is  no  other  tlian  that  of  producing 
a  vacuum  over  one  part  of  the  liquid,  into  which  va- 
cuum the  liquid  is  forced,  by  the  pressure  of  the  atmos- 
phere on  another  part.  The  action  of  sucking  through 
a  straw,  consists  in  drawing  in  and  confining  the 
breath,  so  as  to  produce  a  vacuum  in  the  mouth  ;  in  con- 
sequence of  which,  the  air  within  the  straw  rushes  into 
the  mouth,  and  is  followed  by  the  liquid,  into  which  the 
lower  end  of  the  straw  is  immersed.  The  principle, 
you  see,  is  the  same ;  and  the  only  difference  consists 
in  the  mode  of  producing  a  vacuum.  In  suction,  the 
muscular  powers  answer  the  purpose  of  the  piston  and 
valves. 

Emily.  Water  cannot,  then,  be  raised  by  a  pump 
above  32  feet ;  for  the  pressure  of  the  atmosphere  will 
not  sustain  a  column  of  water  above  that  height. 

Mrs,  B.  1  beg  your  pardon.  It  is  true  that  there 
must  never  be  so  great  a  distance  as  32  feet  from  the 
level  of  the  water  in  the  well,  to  the  valve  in  the  piston, 
otherwise  the  water  would  not  rise  through  that  valve ; 
but  when  once  the  water  has  passed  that  opening,  it  is 
no  longer  the  pressure  of  air  on  the  reservoir  which 
makes  it  ascend  ;  it  is  raised  by  lifting  it  up,  as  you 
would  raise  it  in  a  bucket,  of  which  the  piston  formed  the 
bottom.  This  common  pump  is,  therefore,  called  the 
sucking,  or  lifting-pump,  as  it  is  constructed  on  both 
these  principles.  There  is  another  sort  of  pump,  call- 
ed the  forcing-pump :  it  consists  of  a  forcing  power 
added  to  the  sucking  part  of  the  pump.  This  ad- 
ditional power  is  exactly  on  the  principle  of  the  syringe: 


218  MECHANICAL  PROPERTIES  OP  AIH. 

by  raising  the  piston  jou  draw  the  water  into  the  pump, 
and  by  descending  it  you  force  the  water  out. 

Caroline.  But  the  water  must  be  forced  out  at  the 
upper  part  of  the  pump ;  and  i  cannot  conceive  how 
that  can  be  done  by  descending  the  piston. 

Mrs*  B,  Figure  5.  pi.  XIV.  will  explain  the  difficulty^ 
The  large  pipe  A  B  represents  the  sucking  part  of  the 
pump,  which  differs  from  the  lifting-pump,  only  m  its 
piston  P  being  unfurnished  with  a  valve,  in  consequence 
of  which  the  water  cannot  rise  above  it.  When,  there- 
fore, the  piston  descends,  it  shuts  the  valve  Y,  and 
forces  the  water  (which  has  no  other  vent)  into  the 
pipe  D :  this  is  likewise  furnished  with  a  valve  V, 
which,  opening  outwards,  admits  the  water,  but  pre- 
vents its  return. 

The  water  is  thus  first  raised  in  the  pump,  and  then 
forced  into  the  pipe,  by  the  alternate  ascefiding  and 
descending  motion  of  the  piston,  after  a  few  strokes  of 
the  handle  to  fill  the  pipe,  from  whence  the  wat^r  is- 
sues at  the  spout. 

It  is  now  time  to  conclude  our  lesson.  When  next 
we  meet,  I  shall  give  you  some  account  of  wind,  cxnd 
of  sound,  which  will  terminate  our  observations  on 
elastic  fluids. 

Caroline,  And  I  shall  run  into  the  garden,  to  have 
the  pleasure  of  pumping,  now  that  1  understand  the 
construction  of  a  pump. 

Mrs.  B.  And,  to-morrow  I  hope  you  will  be  able 
to  tell  me,  whether  it  is  a  forcing  or  a  common  lifting 
pump. 


CONVERSA'nON  XIll. 


ON  WIND  AND  SOUND. 

«P     WIXD    IN    GENERAL. OF    THE    TRADE    WIND. OF   THE   PBUfc. 

ODICAL   TRADE    WINDS. OP   THE    AERIAL   TIDES. OF    SOUNDS 

IN    GF.NERAL. OF    SONOROUS    BODIES.       OF    MUSICAL    SOUNDS. 

©F    CONCORD   OR    HARMONY,    AND    MELODY. 


Mrs.  B. 


Well,  Caroline,  have  you  ascertained  what  kind  of 
pump  you  have  in  your  garden  ?    ' 

Caroline,  I  think  it  must  be  merely  a  lifting-pump^ 
because  no  more  force  is  required  to  raise  the  handle 
than  is  necessary  to  lift  its  weight ;  and  in  a  forcing- 
pump,  by  raising  the  handle,  you  force  the  water  into 
the  smaller  pipe,  and  the  resistance  the  water  offers 
must  require  an  exertion  of  strength  to  overcome  it. 

Mrs,  B,  I  make  no  doubt  you  are  right ;  for  lifting 
pumps,  being  simple  in  their  construction,  are  by  far 
the  most  common. 


220  ON  WIND  AND  SOUNB. 

I  have  promised  to  day  to  give  you  some  account  of 
the  nature  of  wind.  Wind  is  nothing  more  than  the 
motion  of  a  stream  or  current  of  air,  generally  produ- 
ced by  a  partial  change  of  temperature  in  the  atmos- 
phere ;  foi  w^hen  any  one  part  is  more  heated  than  the 
rest,  that  part  is  rarefied  ;  the  equilibrium  is  destroyed, 
and  the  air  in  consequence  rises.  When  this  happens, 
there  necessarily  follows  a  motion  of  the  surrounding 
air  towards  that  part,  in  order  to  restore  it ;  this  spot 
therefore,  receives  winds  from  every  quarter.  Those 
who  live  to  the  north  of  it  experience  a  north  wind  ; 
those  to  the  south  a  south  wind  : — do  you  comprehend 
this  ? 

Caroline,  Perfectly.  But  what  sort  of  weather  must 
those  people  have,  who  live  on  the  spot  where  theSe 
winds  meet  and  interfere  ? 

Mrs.  B.  They  have  turbulent  and  boisterous  weather, 
whirlwinds,  hurricanes,  rain,  lightning,  thunder,  &:c. 
This  stormy  weather  occurs  most  frequently  in  the 
torrid  zone,  where  the  heat  is  greatest :  the  air  being 
more  rarefied  there,  than  in  any  other  part  of  the  globe 
is  lighter,  and  consequently  ascends ;  whilst  the  air 
about  the  polar  regions  is  continually  flowing  from  the 
poles  to  restore  the  equilibrium. 

Caroline,  This  motion  of  the  air  would  produce  a 
regular  and  constant  north  wind  to  the  inhabitants  of 
the  northern  hemisphere ;  and  a  south  wind  to  those  of 
the  southern  hemisphere,  and  continual  storms  at  the 
equator,  where  these  two  adverse  winds  would  meet. 

Mrs,  B,  These  winds  do  not  meet,  for  they  each 
change  their  direction  before  they  reach  the  equator. 
The  sun,  in  moving  over  the  equatorial  regions  from 


^N  WIND  AND  SOUND.  S?l 

«a8t  to  west,  rarefies  the  air  as  it  passes,  and  causes 
the  denser  eastern  air  to  flow  westwards,  in  order  to 
restore  the  equilibrium  ;  thus  producing  a  regular  east 
wind  about  the  equator. 

Caroline.  The  air  from  the  west,  then,  constantly 
^oes  to  meet  the  sun,  and  repair  the  disturba;ice  which 
his  beams  have  produced  in  the  equilibrium  of  the  at- 
mosphere. But  r  wonder  how  you  will  reconcile  these 
various  winds,  Mrs.  B. :  you  first  led  me  to  suppose 
there  was  a  constant  struggle  between  opposite  winds 
at  the  equator,  producing  storm  and  tempest ;  but  now 
I  hear  of  one  regular  invariable  wind,  which  must  na- 
turally be  attended  by  calm  weather. 

Emily,  I  think  I  comprehend  it :  do  not  these  winds 
from  the  north  and  south  combine  with  the  easterly 
wind  about  the  equator,  and  form  what  are  called  the 
trade-winds  ? 

Mrs,  B,  Just  so,  my  dear.  The  composition  of  the 
two  winds  north  and  east,  produces  a  constant  north- 
east wind  ;  and  that  of  the  two  winds  south  and  east, 
produces  a  regular  south-east  wind  :  these  winds  ex-- 
tend  to  about  thirty  degrees  on  each  side  of  the  equator, 
the  regions  further  distant  from  it  experiencing  only 
their  respective  north  and  south  winds. 

Caroline.  But  Mrs.  B.,  if  the  air  is  constantly  flow* 
ing  from  the  poles  to  the  torrid  zone,  there  must  be  a 
deficiency  of  air  in  the  polar  regions  ? 

Mrs.  B.  The  light  air  about  the  equator,  which  ex- 
pands and  rises  into  the  upper  legions  of  tl'.e  atmos- 
piiere,  ultimately  flows  from  thence  back  to  the  poles, 
to  restore  the  equilibrium  :  if  it  were  not  for  this  re- 
source, the  polar  atmospheric  regions  would  soon  b& 

T  2 


222  ON  WIND  AND  SOUND. 

exhausted  by  the  stream  of  air,  which,  in  the  lower 
strata  of  the  atmosphere,  thej  are  constantly  sending 
towards  the  equator. 

Caroline.  There  is  then  a  sort  of  circulation  of  air 
in  the  atmosphere ;  the  air  in  the  lower  strata  flowing 
from  the  poles  towards  the  equator,  and  in  the  upper 
strata,  flowing  back  from  the  equator  towards  the  poles. 

Mrs,  B,  Exactly :  I  can  show  you  an  example  of 
this  circulation  on  a  small  scale.  The  air  of  this  room 
being  more  rarefied  than  the  external  air,  a  wind  or 
current  of  air  is  pouring  in  from  the  crevices  of  the 
windows  and  doors,  to  restore  the  equilibrium  ;  but  the 
light  air  with  which  the  room  is  filled  must  find  some 
vent,  in  order  to  make  way  for  the  heavy  air  which  enters. 
If  you  set  the  door  a-jar,  and  hold  a  candle  near  the  up- 
per part  of  it,  you  will  find  that  the  flame  will  be  blown 
outwards,  showing  that  there  is  a  current  of  air  flowing- 
out  from  the  upper  part  of  the  room. — Now  place  the 
candle  on  the  floor  close  by  the  door,  and  you  will  per- 
ceive, by  the  incliriation  of  the  flame,  that  there  is  also 
a  current  of  air  setting  into  the  room. 

Caroline,  It  is  just  so;  the  upper  current  is  the 
warm  light  air,  which  is  driven  out  to  make  way  for  the 
stream  of  cold  dense  air  which  enters  the  room  lower 
down. 

Emily,  I  have  heard,  Mrs.  B.,  that  the  periodical 
winds  are  not  so  regular  on  land  as  at  sea :  what  is  the 
reason  of  that? 

Mrs,  B.  The  land  reflects  into  the  atmosphere  a 
much  greater  quantity  of  the  sun's  rays  than  the  water ; 
therefore,  that  part  of  the  atmosphere  which  is  over  the 
land,  is  more  heated  and  rarefied  than  that  which  is 


ON  WIND  AND  SOUND.  223 

over  the  sea  :  this  occasions  the  wind  to  set  in  upon  the 
land,  as  we  find  that  it  regularly  does  on  the  coast  of 
Guinea,   and  other  countries  in  the  torrid  zone. 

Emily,  1  have  heard  much  of  the  violent  tempests 
occasioned  by  the  breaking  up  of  the  monsoons ;  are 
not  they  also  regular  trade-winds  ? 

•Mrs,  B,  They  are  called  periodical  trade-winds,  as 
they  change  their  course  every  half-year.  This  varia- 
tion is  produced  by  the  earth's  annual  course  round  the 
sun,  when  the  north  pole  is  inclined  towards  that  lumi- 
nary one  half  of  the  year,  the  south  pole  the  other  half. 
During  the  summer  of  the  northern  hemisphere,  the 
countries  of  Arabia,  Persia,  India,  and  China,  are  much 
heated,  and  reflect  great  quantities  of  the  sun's  rays  into 
the  atmosphere,  by  which  it  becomes  extremely  rarefied, 
and  the  equilibrium  consequently  destroved.  In  order  to 
restore  it  the  air  from  the  equatorial  southern  regions, 
where  it  is  colder,  (as  well  as  from  the  colder  northern 
parts,)  must  necessarily  have  a  motion  towards  those 
parts.  The  current  of  air  from  the  equatorial  regions 
produces  the  trade-winds  for  the  first  six  months,  in  all 
the  seas  between  the  heated  continent  of  Asia,  and  the 
equator.  The  other  six  months,  when  it  is  summer  in 
the  southern  hemisphere,  the  ocean  and  countries  to- 
wards the  southern  tropic  are  most  heated,  and  the  air 
over  those  parts  most  rarefied  :  then  the  air  about  the 
equator  alters  its  course,  and  flows  exactly  in  an  opposite 

direction. 

Caroline,  This  explanation  of  the  monsoons  is  very 
curious  ;  but  what  does  their  breaking  up  mean  ? 

Mrs,  B,  It  is  the  name  given  by  sailors  to  the  shift- 
ing of  the  periodical  winds ;  they  do  not  change  their 


$24  ON  WIND  AND   SOUND. 

course  suddenly,  but  by  degrees,  as  the  sun  moves  from 
one  hemisphere  to  the  other :  this  change  is  usually  at- 
tended by  storms  and  hurricanes,  very  dan^^erous  for 
shipping  ;  so  that  those  seas  are  seldom  navigated  at 
the  season  of  the  equinox. 

Emily,  I  think  I  understand  the  winds  in  the  torrid 
zone  perfectly  well  ;  but  what  is  it  that  occasions  the 
great  variety  of  winds  which  occur  in  the  temperate 
zones  ?  for,  according  to  your  theory  there  should  be 
only  north  and  south  winds  in  those  climates. 

Mrs,  B.  Since  so  large  a  portion  of  the  atmosphere 
as  is  over  the  torrid  zone  is  in  continued  agitation,  these 
agitations  in  an  elastic  fluid,  which  yields  to  the 
slightest  impression,  must  extend  every  way  to  a  ^reat 
distance ;  the  air,  therefore,  in  all  climates,  will  suf- 
fer more  or  less  perturbation,  according  to  the  situation 
of  the  country,  the  position  of  mountains,  valleys,  and 
a  variety  of  other  causes  :  hence  it  is  easy  to  conceive, 
that  almost  every  climate  must  be  liable  to  variable 
winds. 

On  the  sea-shore,  there  is  almost  always  a  gentle  sea- 
breeze  setting  in  on  the  land  on  a  summer's  evening,  to 
restore  the  equilibrium  which  had  been  disturbed  hy  re- 
flections from  the  heated  surface  of  the  shore  during  the 
day ;  and  when  night  has  cooled  the  land,  and  con- 
densed the  air,  we  generally  find  it  towards  morning, 
flowing  back  towards  the  sea. 

Caroline.  I  have  observed,  that  the  wind,  which  ever 
way  it  blows,  almost  always  falls  about  sun-set. 

Mrs,  B,  Because  the  rarefaction  of  air  in  the  par- 
ticular spot  which  produces  the  wind,  diminishes  as  the 


ON  WIND   AND  SOUND.  225 

gun  declines,  and  consequently  the  velocity  of  the  wind 
abates. 

Emily.  Since  the  air  is  a  gravitating  fluid,  is  it  not 
affected  by  the  attraction  of  the  moon  and  the  sun,  in 
the  same  manner  as  the  waters  ? 

Mrs.  B.  Undoubtedly  ;  but  the  aerial  tides  are  as 
much  greater  than  those  of  water,  as  the  density  of 
water  exceeds  that  of  air,  which,  as  you  may  recollect^ 
we  found  to  be  about  800  to  1. 

Caroline.  What  a  prodigious  protuberance  that  must 
•ccasion ;  How  much  the  vvpio;ht  of  such  a  column  of 
air  must  raise  the  mercury  in  the  barometer  ! 

Emily,  As  this  enormous  tide  of  air  is  drawn  up 
and  supported,  as  it  were  by  the  moon,  its  weight  and 
pressure,  I  should  suppose,  would  be  rather  diminished 
than  increased  ? 

Mrs.  B.  The  weight  of  the  atmosphere  is  neither 
increased  nor  diminishe*!  by  the  aerial  tides.  The  moon's 
attraction  augments  the  bulk  as  much  as  it  diminishes 
the  weight  of  the  column  of  air ;  these  effects,  therefore, 
counterbalancing  each  other,  the  aerial  tides  do  not  af- 
fect the  barometer. 

Caroline.     I  do  not  quite  understand  that. 

Mrs.  B.  Let  us  suppose  that  the  additional  bulk  of  air 
at  high  tide  raises  the  barometer  one  inch  ;  and  on  the 
other  hand,  that  the  support  which  the  moon's  attraction 
affords  the  air  diminishes  its  weight  or  pressure,  so  as  to 
occasion  the  mercury  to  fall  one  inch  ;  under  these  cir- 
cumstances the  mercury  must  remain  stationary.  Thus 
you  see,  that  we  can  never  be  sensible  of  aerial  tides 
by  the  barometer,  on  account  of  the  equality  of  pres* 
sure  of  the  atmosphere,  whatever  be  its  height. 


226  ON  WIND  AND  SOCND. 

The  existence  of  aerial  tides  is  not,  however,  hy- 
pothetical;  it  is  proved  by  the  effect  they  produce  on 
the  apparent  position  of  the  heavenly  bodies ;  but  this 
I  cannot  explain  to  you,  till  you  understand  the  prOr 
perties  of  light. 

Emili/.     And  when  shall  we  learn  them  ?  ^, 

Mrs,  B.  r shall  first  explain  to  you  the  nature  of 
sound,  which  is  intimately  connected  with  that  of  air  ; 
and  I  think  at  our  next  meeting  we  may  enter  upon  the- 
subject  of  optics. 

We  have  now  considered  the  effects  produced  by  the 
wide  and  extended  aj^itation  of  the  air  ;  but  there  is 
another  kind  of  agitation  of  which  the  air  is  susceptible 
—a  sort  of  vibratory  trembling  motion,  which,  striking 
on  the  drum  of  the  ear  prwluces  sowwc?. 

Caroline.  Is  not  sound  produced  by  solid  bodies? 
The  voice  of  animals,  the  ringing  of  bells,  musical  in- 
struments, are  all  solid  bodies.  I  know  of  no  sound  but 
that  of  the  wind  which  is  produced  by  the  air. 

Mrs,  B.  Sound  I  assure  you,  results  from  a  tremu- 
lous motion  of  the  air  ;  and  the  sonorous  bodies  you  enu- 
merate, are  merely  the  instruments  by  which  that  pe- 
culiar species  of  motion  is  communicated  to  the  air. 

Caroline,  What !  when  l  ring  this  little  bell,  is  it  the 
air  that  sounds,  and  not  the  bell  r 

Mrs.  B.  Both  the  bell  and  the  air  are  concerned  in 
the  production  of  sound.  But  sound,  strictly  speak- 
ing, is  a  perception  excited  in  the  mind  by  the  motion 
of  the  air  on  the  nerves  of  the  ear  ;  the  air,  therefore, 
as  well  as  the  sonorous  bodies  which  put  it  in  motion, 
is  only  the  cause  of  sound,  the  immediate  effect  is  pro- 


ON  WIND  AND  SOUND.  227 

i3uced  by  the  sense  of  hearing:  for  without  this  sense, 
there  would  be  no  sound. 

Emily,  lean  with  diHTiculty  conceive  that.  A  per- 
son born  deaf,  it  is  true,  has  no  idea  of  sound,  because 
he  hears  none  ;  vet  that  does  not  prevent  the  n^al  ex- 
istence of  sound,  as  all  those  who  are  not  deaf  can 
testify. 

Mrs,  B,  I  do  not  doubt  the  existence  of  sound  to 
all  those  who  possess  the  sense  of  hearing  ;  but  it  exists, 
neither  in  the  sonorous  body  nor  in  the  air,  but  in  the 
mind  of  the  person  whose  ear  is  struck  by  the  vibratory 
motion  of  the  air,  produced  by  a  sonorous  body. 

To  convince  you  that  sound  does  not  exist  in  sonor- 
ous bodies,  but  that  air  or  some  other  vehicle  is  necessa- 
ry to  its  production,  endeavour  to  ring  the  little  bell,  af- 
ter I  have  suspended  it  under  a  receiver  in  the  air-pump, 
from  which  I  shall  exhaust  the  air 

Carolint',  This  is  indeed  very  strange  :  though  I 
agitate  it  so  violently,  it  does  not  produce  the  least 
sound. 

Mrs.  B,  By  exhausting  the  receiver,  I  have  cut  off 
the  communication  between  the  air  and  the  bell ;  the 
latter,  therefore,  cannot  impart  its  motion  to  the  air. 

Caroline,  Are  you  sure  that  it  is  not  the  glass,  which 
covers  the  bell,  that  prevents  our  hearing  it. ^'  * 

Mrs.  B,  That  you  may  easily  ascertain  by  letting 
the  air  into  the  receiver,  and  then  ringing  the  belL 

Caroline,  Very  true  :  I  can  hear  it  now  almost  as 
loud  as  if  the  glass  did  not  cover  it ;  and  I  can  no  longer 
doubt  but  that  air  is  necessary  to  the  production  of 
sound.  i-}  ?.,, 


228  ON  WEND  AND  SOtTND. 

Mrs.  B.  Not  absolutely  necessary,  though  by  tar 
the  most  common  vehicle  of  sound.  Liquids,  as  well  as 
air  are  capable  of  conveying  the  vibratory  motion  of  a 
sonorous  body  to  the  organ  of  hearing  ;  as  sound  can 
be  heard  under  water.  Solid  bodies  also  convey  sound, 
as  I  can  soon  convince  you  by  a  very  simple  experiment 
I  shall  fasten  this  string  by  the  middle  round  the  poker; 
now  raise  the  poker  from  the  ground  by  the  two  ends 
of  the  string  and  hold  one  to  each  of  your  ears : — I  shall 
now  strike  the  poker  with  a  key,  and  you  will  find 
that  the  sound  is  conveyed  to  the  ear  by  means  of  the 
strings,  in  a  much  more  perfect  manner  than  if  it  had  no 
other  vehicle  than  the  air. 

Caroline,  That  it  is,  certainly,  for  I  am  almost  stun- 
ned by  the  noise.  But  what  is  a  sonorous  body,  Mrs.  B  ? 
for  all  bodies  are  capable  of  producing  some  kind  of 
sound  by  the  motion  they  communicate  to  the  air. 

Mrs.  B.  Those  bodies  are  called  sonorous,  which 
produce  clear,  distinct,  regular  and  durable  sounds, 
such  as  a  bell,  a  drum,  musical  strings,  wind-instru- 
ments, &c.  They  owe  this  property  to  their  elasticity  ; 
foi  an  elastic  body,  after  having  been  struck,  not  only 
returns  to  its  former  situation,  but  having  acquired  mo* 
mentum  by  its  velocity,  like  the  pendulum,  it  springs 
out  ,on  the  opposite  side.  If  I  draw  the  string  A  B, 
which  is  made  fast  at  both  ends  to  C,  it  will  not  only  re- 
turn to  its  original  position,  but  proceed  onwards  to  D. 
This  is  its  first  vibration,  at  the  end  of  whi'^h  it  will 
retain  sufficient  velocity  to  bring  it  to  E,  and  back  again 
to  F  which  constitutes  its  second  vibration  ;  the  third 
vibration  will  carry  it  onl}'  to  G  and  H,  and  so  on  till 
the  resistance  of  the  air  destroys  its  motion^ 


ON  WIND  AND  SOUND.  229 

The  vibration  of  a  sonorous  body  gives  a  tremulous 
motion  to  the  air  around  it,  very  similar  to  the  motion 
communicated  to  smooth  water  when  a  stone  is  thrown 
into  it.  This  first  produces  a  small  circular  wave  around 
the  spot  in  which  the  stone  falls ;  the  wave  spreads, 
and  gradually  communicates  its  motion  to  the  a<ljacent 
waters,  producing  similar  waves  to  a  considerable  ex- 
tent. The  same  kind  of  waves  are  produced  in  the  air 
by  the  motion  of  a  sonorous  body,  but  with  this  diifer- 
ence,  that  as  air  is  an  elastic  fluid,  the  motion  does 
not  consist  of  regularly  extending  waves,  but  of  vibra- 
tions, and  are  composed  of  a  motion  forwards  and  back- 
wards, similar  to  those  of  the  sonorous  body.  They 
differ  also  in  the  one  taking  place  in  a  plane,  the  other 
in  all  directions.  The  atrial  undulations  being  sphe- 
rical. 

Emily.  But  if  the  air  moves  backwards  as  well  as 
forwards  how  can  its  motion  extend  so  as  to  convey 
sound  to  a  distance. 

Mrs,  B,  The  first  sphere  of  undulations  which  are 
produced  immediately  around  the  sonorous  body,  by 
pressing  against  the  contiguous  air,  condenses  it.  The 
condensed  air,  though  impelled  forward  by  the  pres- 
sure, re-acts  on  the  first  set  of  undulations,  driving 
them  back  again.  The  second  set  of  undulations  which 
have  been  put  in  motion,  in  their  turn  communicate 
their  motion,  and  are  themselves  driven  back  oy  re- 
action. Thus  there  is  a  succession  of  waves  in  the  air, 
corresponding  with  the  succession  of  waves  in  the 
water. 

Caroline.  The  vibrations  of  sound  must  extend  much 


230  .     ON  WIND  AND  SOUND. 

further  than  the  circular  waves  in  water,  since  sound  i& 
conveyed  to  a  great  distance. 

Mrs,  B.  The  air  is  a  fluid  so  much  less  dense  than 
water,  that  motion  is  more  easily  communicated  to  it. 
The  report  of  a  cannon  produces  vibrations  of  the  air 
which  extend  to  several  miles  around. 

Emily,  Distant  sound  takes  some  time  to  reach  us, 
since  it  is  produced  at  the  moment  the  cannon  is  fired  ; 
and  we  see  the  light  of  the  flash  long  .before  we  hear  the 
report. 

Mrs,  B,  The  air  is  immediately  put  in  motion  by 
the  firing  of  a  cannon  ;  but  it  requires  time  for  Ihe  vi- 
brations to  extend  to  any  distant  spot.  The  velocity 
of  sound  is  computed  to  be  at  the  rate  of  1142  feet  in  a 
second. 

Caroline,  With  what  astonishing  rapidity  the  vi- 
brations must  be  communicated !  But  the  velocity  of 
sound  varies,  I  suppose,  with  that  of  the  air  which  con- 
veys it.  If  the  wind  sets  towards  us  from  the  cannon, 
we  iHust  hear  the  report  sooner  than  if  it  set  the  other 
way. 

Mrs,  B,  The  direction  of  the  wind  makes  less  dif- 
ference in  the  velocity  of  sound  than  you  would  imagine. 
If  the  wind  sets  from  us,  it  bears  most  of  the  aerial 
waves  away,  and  renders  the  sound  fainter ;  but  it  is  not 
very  considerably  longer  in  reaching  the  ear  than  if  the 
wind  blew  towards  us.  This  uniform  velocity  of  sound 
enables  us  to  determine  the  distance  of  the  object  from 
which  it  proceeds ;  as  that  of  a  vessel  at  sea  firing  a  can- 
non, or  that  of  a  thunder  cloud.  If  we  do  not  hear  the 
thunder  till  half  a  minute  after  we  see  the  lightnin*',  we 


ON  WIND  AND  SOUND.  231 

eonclude  the  cloud  <o  be  at  the  distance  of  six  miles 
and  a  half. 

Emily,     Pray  how  is  the  sound  of  an  echo  produced  ? 

Mr^,  B,  When  the  aerial  vibrations  meet  with  an 
obstacle,  having  a  hard  and  regular  surface,  such  as  a 
wall,  or  rock,  thej  are  reflected  back  to  the  ear,  and 
produce  the  same  sound  a  second  time  ;  but  the  sound 
will  then  appear  to  proceed  from  the  object  by  which  it 
is  j'eflected.  If  the  vibrations  fall  perpendicularly  on 
the  obstacle,  they  are  reflected  back  in  the  same  line  ; 
if  obliquely,  the  sound  returns  obliquely  in  the  opposite 
direction,  the  angle  of  reflection  being  equal  to  the  an- 
gle of  incidence. 

Caroline,  Oh,  then,  Emily,  I  now  understand  why 
the  echo  of  my  voice  behind  our  house  is  heard  so  much 
plainer  by  you  than  it  is  by  me,  when  we  stand  at  the 
opposite  ends  of  the  gravel  walk.  My  voice,  or  rather, 
I  should  say,  the  vibrations  of  air  it  occasions,  fall 
obliquely  on  the  wall  of  the  house,  and  are  reflected  by 
it  to  the  opposite  end  of  the  gravel  walk. 

Emily,  Very  true ;  and  we  have  observed,  that 
w^hen  we  stand  in  the  middle  of  the  walk,  opposite  the 
house,  the  echo  returns  to  the  person  who  spoke. 

Mrs,  B,  Speaking-trumpets  are  constructed  on  the 
principle  of  the  reflection  of  sound.  The  voice,  instead 
of  being  diffused  in  the  open  air,  is  confined  within  the 
trumpet;  and  the  vibrations,  which  spread  and  fall  a- 
gainst  the  sides  of  the  instrument,  are  reflected  accord- 
ing to  the  angle  of  incidence,  and  fall  into  the  direction 
of  the  vibrations  which  proceed  straight  forwards.  The 
whole  of  the  vibrations  are  thus  collected  into  a  focus; 
and  if  the  ear  be  situated  in  or  near  that  spot,  the  sound 


5  >2  ON  WIND  AND  SOUND. 

is  prodigiously  increased.  Figure  7.  plate  XIV,  will 
give  you  a  clearer  idea  of  the  speaking-trumpet:  the 
reflected  rays  are  distinguished  from  those  of  inci- 
dence, by  being  dotted ;  and  they  are  brought  to  a  focus 
at  F.  The  trumpet  used  by  deaf  persons  acts  on  the 
same  principle  ;  but  as  the  voice  enters  the  trumpet  at 
the  large,  instead  of  the  small  end  of  the  instrument,  it 
is  not  so  much  confined,  nor  the  sound  so  much  in- 
creased. 

Emily.  Are  the  trumpets  used  as  musical  instru- 
ments also  constructed  on  this  principle? 

Mrs,  IL  So  far  a^  their  fotm  tends  to  increase  the 
sound,  they  are  ;  but,  as  a  musical  instrument,  the  trum- 
pet becomes  itself  the  sonorous  body,  which  is  made  to 
vibrate  by  blowing  into  it,  and  communicates  its  vibra- 
tions to  the  air. 

I  will  attempt  to  give  you,  in  a  few  words,  some  no- 
tion of  the  nature  of  musical  sounds,  which  as  you  are 
fond  of  music,  must  be  interesting  to  you. 

If  a  sonorous  body  be  struck  in  such  a  manner,  that 
its  vibrations  are  all  performed  in  regular  times,  the  vi- 
brations of  the  air  will  correspond  with  them  ;  and  strik- 
ing in  the  same  regular  manner  on  the  drum  of  the  ear, 
will  produce  the  same  uniform  sensation  on  the  auditory 
nerve  and  excite  the  same  uniform  idea  in  the  mind ;  or, 
in  other  words,  we  shall  hear  one  musical  tone. 

But  if  the  vibrations  of  the  sonorous  body  are  irregu- 
lar, there  will  necessarily  follow  a  confusion  o/  aerial 
vibrations ;  for  a  second  vibration  may  commence  be- 
fore the  first  is  finished,  meet  it  half  way  on  its  return^ 
interrupt  it  in  its  course,  and  produce  harsh  jarring 
sounds  which  are  called  discords. 


ON  WIND  AND  SOUND.  233 

Emily.  But  each  set  of  these  irregular  vibrations, 
if  repeated  at  equal  intervals,  would,  I  suppose,  pro- 
duce a  musical  tone  ?  It  is  only  their  irregular  succes- 
sion which  makes  them  interfere,  and  occasions  discord. 

Mrs,  B,  Certainly.  The  quicker  a  sonorous  body 
vibrates,  the  more  acute,  or  sharp,  is  the  sound  pro- 
duced. 

Caroline,  But  if  I  strike  any  one  note  of  the  piano- 
forte repeatedly,  whether  quickly  or  slowly,  it  always 
gives  the  same  tone. 

Mrs.  B.  Because  the  vibrations  of  the  same  string, 
at  the  same  degree  of  tension,  are  always  of  a  similar 
duration.  The  quickness  or  slowness  of  the  vibrations 
relate  to  the  single  tones,  not  to  the  various  sounds 
which  they  may  compose  by  succeeding  each  other. 
Striking  the  note  in  quick  succession,  produces  a  more 
frequent  repetition  of  the  tone,  but  does  not  increase 
the  velocity  of  the  vibrations  of  the  string. 

The  duration  of  the  vibrations  of  strings  or  chords 
depends  upon  their  length,  their  thickness  or  weight, 
and  their  degree  of  tension :  thus,  you  find,  the  low 
bass  notes  are  produced  by  long,  thick,  loose  strings ; 
and  tJie  high  treble  notes  by  short,  small,  and  tight 
strings. 

Caroline,  Then  the  different  length  and  size  of  the 
strings  of  musical  instruments,  serves  to  vary  the  du- 
ration of  the  vibrations,  and  consequently,  the  acute- 
ness  of  gravity  of  the  notes  ? 

Mrs,  B,  Yes.  Among  the  variety  of  tones,  there 
are  some  which,  sounded  together,  please  the  ear,  pro- 
ducing what  we  call  harmony,  or  concord.  This  arises 
from  the  agreement  of  the  vibrations  of  the  two  MHior- 


234  QN  WIND  AND  SOUND. 

ous  bodies  ;  so  that  some  of  the  vibrations  of  each  strike 
upon  the  ear  at  the  same  time.  Thus,  if  the  vibrations  of 
two  strings  are  performed  in  equal  times,  the  same  tone 
is  produced  by  both,  and  thej  are  said  to  be  in  unison, 

Emily.  Now,  then,  I  understand  why,  when  I  tune 
my  harp  in  unison  with  the  piano-forte,  I  draw  the 
strings  tighter  if  it  is  too  low,  or  loosen  them  if  it  is 
at  too  high  a  pitch  :  it  is  in  order  to  bring  them  to  vi- 
brate, in  equal  times,  with  the  strings  of  the  piano- 
forte. 

Mrs,  B.  But  concord,  you  know,  is  not  confined  to 
unison ;  for  two  different  tones  harmonize  in  a  variety 
of  cases.  If  the  vibrations  of  one  string  (or  sonorous 
body  whatever)  vibrate  in  double  the  time  of  another, 
the  second  vibration  of  the  latter  will  strike  upon  the 
ear  at  the  same  instant  as  the  first  vibration  of  the  for- 
mer ;  and  this  is  the  concord  of  an  octave. 

If  the  vibrations  of  two  strings  are  as  two  to  three 
the  second  vibration  of  the  first  corresponds  with  the 
third  vibration  of  the  latter,  producing  the  harmony 
called  a  fifth. 

Caroline.  So,  then,  when  I  strike  the  key-note  with 
its  fifth,  I  hear  every  second  vibration  of  one,  and  eve- 
ry third  of  the  other  at  the  same  time  ? 

Mrs,  B,  Yes;  and  the  key-note  struck  with  the 
fourth  is  likewise  a  concord,  because  the  vibrations  are 
as  three  to  four.  The  vibrations  of  a  major  third  with 
the  key-note,  are  as  four  to  five  ',  and  those  of  a  mi- 
nor third,  as  five  to  six. 

There  are  other  tones  which,  though  they  cannot  be 
struck  together  without  producing  discord,  if  struck 
snecessively,  gives  us  the  pleasure  which  is  called  me- 


ON  WIND  AND  SOUND.  235 

lodj.  Upon  these  general  principles  the  science  of 
music  is  founded  ;  but  I  am  not  sufficiently  acquainted 
with  it  to  enter  arny  further  into  it. 

We  shall  now,  therefore,  take  leave  of  the  subject 
of  sound  ;  and,  at  our  next  interview,  enter  upon  that 
of  optics,  in  which  we  shall  consider  the  nature  of  vi- 
sion, light,  and  colours. 


CONVERSATION  XIT. 


ON  OPTICS. 

OF  LUMINOUS,   THANSPARENT,    ANP    OPAaUE    BODIES. — OF  THE  HA- 

BIATIOir   OP    LIGHT. — OF    SHADOWS. OF   THE     HEFLECTION     OF 

LIGHT. OPAaUE    BODIES    SEEN    ONLY    BT  REFLECTED    LIGHT. 

VISION  EXPLAINED. — CAMERA  OBSCURA. IMAGE    OF  OBJECTS  ON 

THE    RETINA. 


Caroline. 

1  LONG  to  begin  our  lesson  to  day,  Mrs.  B.,  for  I  ex- 
pect that  it  will  be  very  entertaining. 

Mrs.  B.  Optics  is  certainly  one  of  the  most  inte- 
resting branches  of  Natural  Philosophy,  but  not  one  of 
the  easiest  to  understand ;  I  must  therefore  beg  that 
you  will  give  me  the  whole  of  your  attention. 

I  shall  first  inquire,  whether  you  comprehend  the 
meaning  of  a  luminous  body,  an  opaque  body,  and  a 
transparent  body. 

Caroline,  A  luminous  body  is  one  that  shines ;  an 
opaque .... 


238  ON  OPTICS. 

Mrs,  B.  Do  not  proceed  to  the  second,  until  we 
have  agreed  upon  the  definition  of  the  first.  k\\  bodies 
that  shine  are  not  luminous;  for  a  luminous  body  is 
one  that  shines  by  its  own  light,  as  the  sun,  the  fire,  a 
candle,  &c. 

Emily.  Polished  metal  then,  when  it  shines  with  so 
much  brilliancy,  is  not  a  luminous  body  ? 

Mrs.  B.  No,  for  it  would  be  dark  if  it  did  not  re- 
ceive light  from  a  luminous  body  ;  it  belongs,  therefore, 
to  the  class  of  opaque  or  dark  bodies,  which  compre- 
hend all  such  as  are  neither  luminous  nor  will  admit  the 
light  to  pass  through  them. 

Emily.  And  transparent  bodies,  are  those  which 
admit  the  light  to  pass  through  them  ;  such  as  glass  and 
water  ? 

Mrs.  B.  You  are  right.  Transparent  or  pellucid 
bodies,  are  frequently  called  mediums  ;  and  the  rays  of 
light  which  pass  through  them,  are  said  to  be  transmit- 
ted by  them. 

Light,  when  emanated  from  the  sun,  or  any  other 
luminous  body,  is  projected  forwards  in  straight  lines 
in  every  possible  direction ;  so  that  the  luminous  body 
is  not  only  the  general  centre  from  whence  all  the  rays 
proceed ;  but  every  point  of  it  may  be  considered  as  a 
centre  which  radiates  light  in  every  direction,  (fig.  1. 
plate  XV.) 

Emily.  But  do  not  the  the  rays  which  are  projected 
in  different  directions,  and  cross  each  other,  interfere 
and  impede  each  other's  course  ? 

Mrs.  B.  Not  at  all.  The  particles  of  light  are  so 
extremely  minute,  that  they  are  never  known  to  inter- 
fere with  each  other.     A  ray  of  light  is  a  single  line  of 


Jhih.  Try  J.TStmtphr-ejs  27n7nd'^ 


ON  OPTICS.  239 

light  projected  from  a  luminous  body;  and  a  pencil  of 
rays,  is  a  collection  of  rajs,  proceeding  from  any  one 
point  of  a  luminous  body,  as  fig.  2. 

Caroline.  Is  light  then  a  substance  composed  of 
particles  like  other  bodies  ? 

Mrs.  B.  This  is  a  disputed  point,  upon  which  I  can- 
not pretend  to  decide.  In  some  respects,  light  is  obe- 
dient to  the  laws  which  govern  bodies  ;  in  others,  it  ap- 
pears to  be  independent  of  them  :  thus  though  its  course 
is  guided  by  the  laws  of  motion,  it  does  not  seem  to 
be  influenced  by  those  of  gravity.  It  has  never  been 
discovered  to  have  weight,  though  a  variety  of  interest- 
ing experiments  have  been  made  with  a  view  of  ascer- 
taining that  point ;  but  we  are  so  ignorant  of  the  inti- 
mate nature  of  light,  that  an  attempt  to  investage  it 
would  lead  us  into  a  labyrinth  of  perplexity,  if  not  of 
error  ;  we  shall  therefore  confine  our  attention  to  those 
properties  of  light  which  are  well  ascertained. 

Let  us  return  to  the  examination  of  the  effects  of  the 
radiation  of  light  from  a  luminous  body.  Since  the  rays 
of  light  are  projected  in  straight  lines,  when  they  i\ieet 
with  an  opaque  body  through  which  they  are  unable  to 
pass,  they  are  stopped  short  in  their  course  ;  for  they 
cannot  move  in  a  curve  line  round  the  body. 

Caroline.  No,  certainly;  for  it  would  require  some 
other  force  besides  that  of  projection,  to  produce  mo- 
tion in  a  curve  line. 

JVr.s.  B.  The  interruption  of  the  rays  of  light,  by 
the  opaque  body,  protluces,  therefore,  darkness  on  the 
opposite  side  of  it ;  and  if  this  darkness  fall  upon  a  wall, 
a  j^heet  of  paper,  or  any  object  whatever,  it  forms  a 
shadow. 


240  ON  OPTICS. 

Emily,  A  shadow  then  is  nothing  more  than  darkness 
produced  by  the  intervention  of  an  opaque  body,  which 
prevents  the  rays  of  light  from  reaching  an  object  behind 
the  opaque  body. 

Caroline,  Why  then  are  shadows  of  different  de- 
grees of  darkness  ;  for  I  should  have  supposed  from 
your  definition  of  a  shadow,  that  it  would  have  been  per- 
fectly black  ? 

Mrs,  B,  It  frequently  happens  that  a  shadow  is 
produced  by  an  opaque  body  interrupting  the  course  of 
the  rays  from  one  luminous  body,  while  light  from  ano- 
ther reaches  the  space  where  the  shadow  is  formed,  in 
which  case  the  shadow  is  proportionally  fainter.  This 
happens  if  the  opaque  body  be  lighted  by  two  candles  : 
if  you  extinguish  one  of  them,  the  shadow  will  be  both 
deeper  and  more  distinct. 

Caroline,     But  yet  it  will  not  be  perfectly  dark. 

Mrs,  B,  Because  it  is  still  slightly  illumined  by  light 
reflected  from  the  walls  of  the  room,  and  other  sur- 
rounding objects. 

You  must  observe,  also,  that  when  a  shadow  is  pro- 
duced by  the  interruption  of  rays  from  a  single  luminous 
body,  the  darkness  is  proportional  to  the  intensity  of 
the  light. 

Emily,  I  should  have  supposed  the  Contrary ;  for  as 
the  light  reflected  from  surrounding  objects  on  the  sha- 
dow, must  be  in  proportion  to  the  intensity  of  the  light, 
the  stronger  the  light,  the  more  the  shadow  will  be  il- 
lumined. 

Mrs,  B,  Your  remark  is  perfectly  just ;  but  as  we 
have  no  means  of  estimating  the  degrees  of  light  and 
of  darkness  but  by  comparison,  the  strongest  light  will 


ON  OPTICS.  241 

appear  to  produce  the  deepest  shadow.  Hence  a  total 
eclipse  of  the  sun  occasions  a  more  sensible  darkness 
than  mid-night,  as  it  is  immediately  contrasted  with  the 
strong  light  of  noon-day. 

Caroline.  The  re-appearance  of  the  sun  after  aa 
eclipse,  must  by  same  contrast  be  remarkably  brilliant. 

Mrs.  B.  Certainly.  There  are  several  things  to  be 
observed  in  regard  to  the  form  and  extent  of  shadows. 
If  the  luminous  body  A  (fig.  3.)  is  larger  than  the  opaque 
body  B,  the  shadow  will  gradually  diminish  in  size,  till 
it  terminate  in  a  point. 

Caroline,  This  is  the  case  with  the  shadows  of  the 
earth  and  the  moon,  as  the  sun  which  illumines  them,  is 
larger  than  either  of  those  bodies.  And  why  is  it  not 
the  case  with  the  shadows  of  terrestrial  objects,  which 
are  equally  illumined  by  the  sun  ?  but  their  shadows, 
far  from  diminishing,  are  always  larger  than  the  object, 
and  increase  with  the  distance  from  it. 

Mrs.  B.  In  estimating  the  effect  of  shadows,  we  must 
consider  the  apparent  not  the  real  dimensions  of  the  lu- 
minous body ;  and  in  this  point  of  view,  the  sun  is  a 
small  object  compared  with  the  generality  of  the  terres- 
trial bodies  which  it  illumines  :  and  when  the  luminous 
body  is  less  than  the  opaque  body,  the  shadow  will  in- 
crease with  the  distance  to  infinity.  All  objects,  there- 
fore, which  are  apparently  larger  than  the  sun,  cast 
a  magnified  shadovv.  This  will  be  best  exemplified,  by 
observing  the  shadow  of  an  object  lighted  by  a  candle. 

Emily.  1  have  often  noticed,  that  the  shadow  of  my 
figure  against  the  wall,  grows  larger  as  it  is  more  dis- 
tant from  me,  which  is  owing,  no  doubt,  to  the  candle 
that  shines  on  me  being  much  smaller  than  myself? 


'24^  ON  OPTICS. 

Mrs.  B,  Yes.  The  shadow  of  a  figure  A,  (fig.  4.) 
varies  in  size,  according  to  the  distance  of  the  several 
surfaces  B  C  D  E,  on  which  it  is  described. 

Caroline,  I  have  observed,  that  two  candles  produce 
two  shadows  from  the  same  object;  whilst  it  would  ap- 
pear, from  what  you  said,  that  they  should  rather  pro- 
duce only  half  a  shadow,  that  is  to  say,  a  very  faint  one. 

Mrs.  B,  The  number  of  lights  (in  different  direc- 
tions) while  it  decreases  the  intensity  of  the  shadow, 
increases  their  number,  which  always  corresponds  with 
that  of  the  lights ;  for  each  light  makes  the  opaque 
body  cast  a  different  shadow,  as  illustrated  by  fig.  5. 
It  represents  a  ball  A,  lighted  by  three  candles  B,  C,  D, 
and  you  observe  the  light  B  produces  the  shadow  b,  the 
light  C  the  shadow  c,  and  the  light  D  the  shadow  d, 

Emily.  1  think  we  now  understand  the  nature  of 
shadows  very  well  ;  but  j^ay  what  becomes  of  the  rays 
of  light  which  opaque  bodies  arrest  In  their  course,  and 
the  interruption  of  which  is  the  occasion  of  shadows  ? 

Mrs,  B,  Your  question  leads  to  a  very  important 
property  of  light,  Reflection,  When  rays  of  light  en- 
counter an  opaque  body,  which  they  cannot  traverse, 
part  of  them  are  absorbed  by  it,  and  part  are  reflected, 
and  rebound  just  as  an  elastic  ball  which  is  struck 
against  a  wall. 

Emily.  And  is  light  in  its  reflection  governed  by  the 
same  laws  as  solid  elastic  bodies  ? 

Mrs.  B.  Exactly.  If  a  ray  of  light  fall  perpendicu- 
larly on  an  opaque  body,  it  is  reflected  back  in  the  same 
line,  towards  the  point  whence  it  proceeded.  If  it  fall 
obliquely,  it  is  reflected  obliquely,  but  in  the  opposite 
direction  ;  the  angle  of  incidence  being  equal  to  the  an- 
gle of  reflection.    You  recollect  that  law  in  mechanics  i 


ON  OPTICS.  243 

Emily*    Oh  yes,  perfectly. 

Mrs.  B,  If  you  will  shut  the  shatters,  we  shall  ad- 
mit a  ray  of  the  sun's  light  through  a  very  small  aper- 
ture, and  I  can  show  you  how  it  is  reflected.  I  now 
hold  this  mirror,  so  that  the  ray  shall  fall  perpen- 
dicularly upon  it. 

Caroline,  I  see  the  ray  which  falls  upon  the  mirror, 
but  not  that  which  is  reflected  by  it. 

Mrs,  B.  Because  its  reflection  is  directly  retro- 
grade. The  ray  of  incidence  and  that  of  reflection 
both  being  in  the  same  line,  though  in  opposite  direc- 
tions, are  confounded  together. 

Emily,  The  ray  then  which  appears  to  us  single, 
is  really  double,  and  is  composed  of  the  incident  ray 
proceeding  to  the  mirror,  and  of  the  reflected  ray  re- 
turning from  the- mirror. 

Mrs,  B,  Exactly  so.  We  shall  now  separate  them 
by  holding  the  mirror  M,  (fig.  6.)  in  such  a  manner,  that 
the  incident  ray  A  B  shall  fall  obliquely  upon  it — you 
see  the  reflected  ray  B  C,  is  marching  off  in  another 
direction.  If  we  draw  a  line  from  the  point  of  inci- 
dence B,  perpendicular  to  the  mirror,  it  will  divide  the 
angle  of  incidence  from  the  angle  of  reflection,  and 
you  will  see  that  they  are  equal. 

Emily.  Exactly  ;  and  now  that  you  hold  the  mirror 
so,  that  the  ray  falls  more  obliquely  on  it,  it  is  also  re- 
flected more  obliquely,  preserving  the  equality  of  the. 
angles  of  incidence  and  reflection. 

Mrs,  B,  It  is  by  reflected  rays  only  that  we  see 
opaque  objects.  Luminous  bodies  send  rays  of  lighting 
mediately  to  our  eyes,  but  the  rays  which  they  send  to 
other  bodies  are  invisible  to  us,  and  are  seen  only  when 
reflected  or  transmitted  by  those  bodies  to  our  eyes. 


244  ON  OPTieS. 

Emily.  But  have  we  not  just  seen  the  ray  of  light 
in  its  passage  from  the  sun  to  the  mirror,  and  its  reflec- 
tion ?  yet  in  neither  case  were  those  rays  in  a  direc- 
tion to  enter  our  eyes. 

Mrs,  B,  No.  What  you  saw  was  the  light  reflect- 
ed to  your  eyes  by  small  particles  of  dust  floating  in  the 
air,  and  on  which  the  ray  shown  in  its  passage  to  and 
from  the  mirror. 

Caroline.  Yet  I  see  the  sun  shining  on  that  house 
yonder,  as  clearly  as  possible. 

Jlrs.  B.  Indeed  you  cannot  see  a  single  ray  which 
passes  from  the  sun  to  the  house ;  you  see  no  rays  but 
those  which  enter  your  eyes  ;  therefore  it  is  the  rays 
which  are  reflected  by  the  house  to  you,  and  not  those 
which  proceed  from  the  sun  to  the  house,  that  are  visi^ 
ble  to  you. 

Caroline.  Why  then  does  one  side  of  the  house  ap- 
pear to  be  in  sunshine,  and  the  other  in  the  shade  ?  for 
if  I  cannot  see  the  sun  shine  upon  it,  the  whole  of  the 
house  should  appear  in  the  shade. 

Mrs,  B.  That  side  of  the  house  which  the  sun  shines 
upon,  reflects  more  vivid  and  luminous  rays  than  the 
side  which  is  in  shadow,  for  the  latter  is  illumined  only 
by  rays  reflected  upon  it  by  other  objects,  these  rays 
are  therefore  twice  reflected  before  they  reach  your 
sight;  and  as  light  is  more  or  less  absorbed  by  the  bodies 
it  strikes  upon,  every  time  a  ray  is  reflected  its  inten- 
sity is  diminished. 

Caroline.  SHll  I  cannot  reconcile  myself  to  the 
idea,  that  we  do  not  see  the  sun's  rays  shining  on  objects, 
but  only  those  which  objects  reflect  to  us. 

Mrs.  B.    I  do  not,  however,  despair  of  convincing 


ON  OPTICS,  245 

you  of  it.  Look  at  that  large  sheet  of  water,  can  you 
tell  why  the  sun  appears  to  shine  on  one  part  of  it  only? 

Caroline.  No,  indeed  ;  for  the  whole  of  it  is  equal- 
ly exposed  to  the  sun.  This  partial  brilliancy  of  water 
has  often  excited  my  wonder ;  but  it  has  struck  me 
more  particularly  by  moon-light.  I  have  frequently 
observed  a  vivid  streak  of  moonshine  on  the  sea,  while 
the  re^t  of  the  water  remained  in  deep  obscurity,  and 
yet  there  was  no  apparent  obstacle  to  prevent  the  moon 
from  shining  on  every  part  of  the  water  equally. 

Mrs,  B.  By  moon-light  the  effect  is  more  remarka- 
ble, on  account  of  the  deep  obscurity  of  the  other  parts 
of  the  water  y  vvhile  by  the  sun's  light  the  effect  is  too 
strong  for  the  eye  to  be  able  to  contemplate  it. 

Caroline,  But  if  the  sun  really  shines  on  every  part 
of  that  sheet  of  water,  why  does  not  every  part  of  it 
reflect  rays  to  my  eyes  ? 

Mrs,  B,  The  reflected  rays  are  not  attracted  out  of 
their  natural  course  by  your  ejes.  The  direction  of  a 
reflected  ray,  you  knov»%  depends  on  that  of  the  inci- 
dent ray  ;  the  sun's  rays,  therefore,  which  fall  with  va- 
rious degrees  of  obliquity  upon  the  water,  are  reflected 
in  directions  equally  various ;  some  of  these  will  meet 
your  eyes,  and  you  will  see  them,  but  those  which  fall 
elsewhere  are  invisible  to  you. 

Caroline,  The  streak  of  sunshine,  then,  which  we 
now  see  upon  the  water,  is  composed  of  those  rays 
which  by  their  reflection  happen  to  fall  upon  my  eyes? 

Mrs,  B,    Precisely. 

Emily,  But  is  that  side  of  the  house  yonder,  which 
appears  to  be  in  shadow,  really  illumined  by  the  sun, 
and  its  rays  reflected  another  way  ? 


-io  ON  OPTICS. 

Mrs.  B,  No  ;  that  is  a  different  case  from  the  sheet 
of  water.  That  side  of  the  hou»e  is  really  in  shadow; 
it  is  the  west  side,  which  the  sun  cannot  shine  upon  till 
the  afternoon. 

Emily.  Those  objects,  then,  which  are  illumined  by 
rellected  rays,  and  those  which  receive  direct  rays  from 
the  sun,  but  which  do  not  reflect  those  rays  towards  ys, 
appear  equally  in  shadow  ? 

Mrs,  B.  Certainly ;  for  we  see  them  both  illumined 
by  reflected  rays.  That  part  of  the  sheet  of  water, 
over  which  the  trees  cast  a  shadow,  by  what  light  cfe 
you  see  it, 

Emily,  Since  it  is  not  by  the  sun's  direct  rays,  it 
must  be  by  those  reflected  on  it  from  other  objects,  and 
which  it  again  reflects  to  us. 

Caroline,  But  if  we  see  all  terrestrial  objects  by  re- 
flected light,  (as  we  do  the  moon,)  why  do  they  appear 
so  bright  and  luminous  ?  1  should  have  supposed  that 
reflected  rays  would  have  been  dull  and  faint,  like  those 
of  the  moon. 

Mrs,  B,  The  moon  reflect^  the  sun's  light  with  as 
much  vividness  as  any  terrestrial  object.  If  you  look 
at  it  on  a  clear  night,  it  w  ill  appear  as  bright  as  a  sheet 
of  water,  the  walls  of  a  house,  or  any  object  seen  by 
daylight  and  on  which  the  sun  shines.  The  rays  of  the 
moon  are  doubtless  feeble,  when  compared  with  those 
of  the  sun;  but  that  would  not  be  a  fair  comparison, 
for  the  former  are  incident,  the  latter  reflected  rays. 

Caroline.  True  ?  and  when  we  see  terrestrial  objects 
by  moon-light,  the  light  has  been  twice  reflected,  and 
is  consequently  proportionally  fainter. 

Mrs,  B,    In  traversing  the  atmosphere,  the  rays, 


ON  OPTICS.  247 

both  of  the  sun  and  moon,  lose  some  of  their  light. 
For  though  the  pure  air  is  a  transparent  medium,  which 
transmits  the  rajs  of  light  freely,  we  have  observed, 
that  near  the  surface  of  the  earth  it  is  loaded  with  va- 
pours and  exhalations,  by  which  some  portion  of  them 
are  absorbed. 

Caroline,  I  have  often  noticed,  that  an  object  on  the 
summit  of  a  hill  appears  more  distinct  than  one  at  an 
equal  distance  in  a  valley,  or  on  a  plain  ;  which  is  owing 
I  suppose,  to  the  air  being  more  free  from  vapours  in  an 
elevated  situation,  and  the  reflected  rays  being  conse- 
quently brighter. 

Mrs.  B.  That  may  have  some  sensible  effect ;  but 
when  an  object  on  the  summit  of  a  hill  has  a  back  ground 
of  light  sky,  the  contrast  with  the  object  makes  its  out- 
line more  distinct. 

Caroline.  I  now  feel  well  satisfied,  that  we  see 
opaque  objects  only  by  reflected  rays;  but  I  do  not  un- 
derstand how  these  rays  show  us  the  objects  from 
which  they   proceed  ? 

Mrs.  B.  The  rays  of  light  enter  at  the  pupil  of  the 
eye,  and  proceed  to  the  retina,  or  optic  nerve  which  is 
situated  at  the  back  part  of  the  eye-ball ;  and  there  they 
describe  the  figure,  colour,  and  (excepting  size)  form  a 
perfect  representation  of  the  object  from  which  they 
proceed.  We  shall  again  close  the  shutters,  and  admit 
the  light  through  the  small  aperture,  and  you  will 
see  a  picture  on  the  wall,  opposite  the  aperture, .simi- 
lar to  that  which  is  delineated  on  the  retina  of  the  eye. 

Caroline.  Oh,  how  wonderful !  There  is  an  exact  pic- 
ture in  miniature  of  the  garden,  the  gardener  at  work, 
the  trees  blown  about  by  tne  wind.  The  landscape  would 


248  ON  OPTICS, 

be  perfect,  if  it  were  not  reversed  ;  the   ground  being 
above,  and  the  sky  beneath. 

Mrs.  B,  It  is  not  enough  to  admire,  you  must  un- 
derstand this  phenomenon,  which  is  called  a  camera  ob- 
scura,  from  the  necessity  of  darkening  the  room,  in  or- 
der to  exhibit  it. 

This  picture  is  produced  by  the  rays  of  light  reflected 
from  the  various  objects  in  the  garden,  and  which  are 
admitted  through  the  hole  in  the  window  shutter. 

The  rays  from  the  glittering  weathercock  at  the  top 
of  the  alcove  A,  (plate  XVI.  fig.  1.)  represent  it  in 
this  spot  a  ;  for  the  weathercock  being  much  higher 
than  the  aperture  in  the  shutter,  only  a  few  of  the  rays, 
which  are  reflected  by  it  in  an  obliquely  descending  di- 
rection, can  find  entrance  there.  The  rays  of  light, 
you  know,  always  move  in  straight  lines  ;  those,  there- 
fore, which  enter  the  room  in  a  descending  direction, 
will  continue  their  course  in  the  same  direction,  and 
will,  consequently,  fall  upon  the  lower  part  of  the  wall 
opposite  the  aperture,  and  represent  the  weathercock 
reversed  in  that  spot,  instead  of  erect  in  the  uppermost 
part  of  the  landscape. 

Emily,  And  the  rays  of  light  from  the  steps  (B)  of 
the  alcove,  in  entering  the  aperture,  ascend,  and  will 
describe  those  steps  in  the  highest  instead  of  the 
lowest  part  of  the  landscape. 

Mrs»  B,  Observe,  too,  that  the  rays  coming  from  the 
alcove,  which  is  to  our  left,  describe  it  on  the  wall  to  the 
right ;  while  those  which  are  reflected  by  the  walnut- 
tree  C  D,  to  our  right,  delineate  its  figure  in  the  pic- 
ture to  the  left  c  d.  Thus  the  rays,  coming  in  different 
directions,  and  proceeding  always  in  right  lines,  cross 


hth.  hy  .IXUruupltj-^ys    Vlalo^l. 


GN  OPTICS.  249 

each  other  at  their  entrance  through  the  aperture :  those 
which  come  above  proceed  below,  those  from  the  right 
go  to  the  left,  those  from  the  left  towards  the  right ; 
thus  every  object  is  represented  in  the  picture,  as  oc- 
cupying a  situation  the  very  reverse  of  that  which  it 
does  in  nature.  '• 

Caroline,  Excepting  the  flower-pot  E  F,  which, 
though  its  position  is  reversed,  has  not  changed  its  situ- 
ation in  the  landscape. 

Mrs,  B,  The  flower-pot  is  directly  in  front  of  the 
aperture;  so  that  its  rays  fall  perpendicularly  upon  it, 
and,  consequently,  proceed  perpendicularly  to  the  wall, 
where  they  delineate  the  object  directly  behind  the 
aperture. 

Emily,  And  is  it  thus  that  the  picture  of  objects  is 
painted  on  the  retina  of  the  eye  ? 

Mrs,  B,  Precisely.  The  pupil  of  the  eye,  through 
which  the  rays  of  light  enter,  represents  the  aperture  in 
the  window-shutter ;  and  the  image  delineated  on  the 
retina,  is  exactly  similar  to  the  picture  on  the  wall. 

Caroline,  You  do  not  mean  to  say,  that  we  see  only 
the  representation  of  the  object  which  is  painted  on  the 
retina,  and  not  the  object  itself? 

Mrs,  B.  If,  by  sight,  you  understand  that  sense  by 
which  the  presence  of  objects  is  perceived  by  the  mind, 
through  the  means  of  the  eyes,  we  certainly  see  only 
the  image  of  those  objects  painted  on  the  retina. 

Caroline,     This  appears  to  me  quite  incredible. 

Mrs,  B,  The  nerves  are  the  only  [  ar  t  of  our  frame 
capable  of  sensation :  they  appear,  therefore,  to  be  the 
instruments  which  the  mind  employs  in  its  perceptions  ; 
for  a  sensation  always  conveys  an  idea  to  the  mind. 


250  ON  OPTICS. 

Now  it  is  known,  that  our  nerves  can  be  affected  only  bj 
contact ;  and  for  this  reason  the  organs  of  sense  cannot 
act  at  a  distance:  for  instance,  we  are  capable  of  smel- 
ling only  particles  which  are  actually  in  contact  with 
the  nerves  of  the  nose.  We  have  already  observed, 
that  the  odour  of  a  flower  consists  in  effluvia.,  composed 
of  very  minute  particles,  which  penetrate  the  nostrils, 
and  strike  upon  tlie  olfactory  nerves,  which  instantly 
con\ey  the  idea  of  smell  to  the  mind. 

Emily.  And  sound,  though  it  is  said  to  be  heard  at 
a  distance,  is,  in  fact,  heard  only  when  the  vibrations  of 
the  air,  which  convey  it  to  our  ears,  strike  upon  the  au- 
ditory nerve. 

Caroline.  There  is  no  explanation  required,  to  prove 
that  the  senses  of  feeling  and  of  tasting  are  excited  only 
by  contact.  » 

Mrs.  B.  And  I  hope  to  convince  you,  that  the  sense 
of  sight  is  so  likewise.  The  nerves,  which  constitute 
the  sense  of  sight,  are  not  different  in  their  nature  from 
those  of  the  other  organs  ;  they  are  merely  instruments 
which  convey  ideas  to'the  mind,  and  can  be  affected  only 
on  contact.  Now,  since  real  objects  cannot  be  brought 
to  touch  the  optic  nerve,  the  image  of  them  is  conveyed 
thither  by  the  rays  of  light  proceeding  from  real  objects, 
which  actually  strike  upon  the  optic  nerve,  and  form 
that  image  which  the  mind  perceives. 

Caroline.  While  I  listen  to  your  reasoning,  I  feel 
convinced  ;  but  when  I  look  upon  the  objects  around, 
and  think  that  I  do  not  see  them,  but  merely  their  image 
painted  in  my  eyes,  my  belief  is  again  staggered.  I 
cannot  reconcile  myself  to  the  idea,  that  1  do  not  really 
sef^  this  book  which  I  hold  in  my  hand,  nor  the  words 
which  1  read  in  it. 


ON  OPTICS.  251 

Mrs,  B.  Did  it  ever  occur  to  you  as  extraordinary, 
that  you  never  beheld  your  own  face  ? 

Caroline.  No ;  because  I  so  frequently  see  an  exact 
x'epresentation  of  it  in  the  looking-glass. 

Mrs,  B.  You  see  a  far  more  exact  representation  of 
objects  on  the  retina  of  your  eye  :  it  is  a  much  more 
perfect  mirror  than  any  made  by  art. 

Emily,  But  is  it  possible,  that  the  extensive  land- 
scape, which  I  now  behold  from  the  window,  should  be 
represented  on  so  small  a  space  as  the  retina  of  the  eye  ? 

Mrs,  B,  It  would  be  impossible  for  art  to  paint  so 
small  and  distinct  a  miniature  ;  but  nature  works  with  a 
surer  hand,  and  a  more  delicate  pencil.  1  hat  power, 
which  forms  the  feathers  of  the  butterfly,  and  thefluu- 
erets  of  the  daisy,  can  alone  pourtray  so  admirable  asid 
perfect  a  miniature  as  that  which  is  represented  on  tlie 
retina  of  the  eye. 

Caroline,  But,  Mrs.  B.,  if  we  see  only  the  image  of 
objects,  why  do  we  not  see  them  reversed,  as  you  show- 
ed us  they  were  in  the  camera  obscura?  Is  not  that  a 
strong  argument  against  your  theory  ? 

Mrs,  B,  Not  ah  unanswerable  one,  I  hope.  The 
image  on  the  retina  it  is  true,  is  reversed,  like  that  in 
the  camera  obscura ;  as  the  rays,  unless  from  a  very 
small  object,  intersect  each  other  on  entering  the  pupil, 
in  the  same  manner  as  they  do  on  entering  the  camera 
obscura.  The  scene,  however,  does  not  excite  the  idea 
of  being  inverted,  because  we  always  see  an  object  in 
the  direction  of  the  rays  which  it  sends  to  us. 

Emily.     I  confess  I  do  not  understand  that. 

Mrs,  B,  It  is,  I  think,  a  difficult  point  to  explain 
clearly.     A  ray  which  comes  from  the  Upper  part  of  an 


252  ON  OPTICS. 

object,  describes  the  image  on  the  lower  part  of  the  re- 
tina ;  but  experience  having  taught  us,  that  the  direc- 
tion of  that  ray  is  from  above,  we  consider  that  part 
of  the  object  it  represents  as  uppermost.  The  rays 
proceeding  from  the  lower  part  of  an  object  fall  upon 
the  upper  part  of  the  retina  ;  but  as  we  know  their  di- 
rection to  be  from  below,  we  see  that  part  of  the  object 
they  describe  as  the  lowest. 

Caroline.  When  I  want  to  see  an  object  above  me, 
I  look  up ;  when  an  object  below  me,  I  look  down.  Does 
not  this  prove  that  1  see  the  objects  the4nselves  ?  for  if 
I  beheld  only  the  image,  there  would  be  no  necessity 
for  looking  up  or  down,  according  as  the  object  was 
higher  or  lower  than  myself. 

Mrs.  B.  I  beg  jour  pardon.  When  you  look  up  to 
an  elevated  object,  it  is  in  order  that  the  rays  reflected 
from  it  should  fall  upon  the  retina  of  your  eyes;  but 
the  very  circumstance  of  directing  your  eyes  upwards 
convinces  you  that  the  object  is  elevated,  and  teaches 
you  to  consider  as  uppermost  the  image  it  forms  on  the 
retina,  though  it  is,  in  fact,  represented  in  the  lowest 
part  of  it.  When  you  look  down  upon  an  object,  you 
draw  your  conclusion  from  a  similar  reasoning;  it  is 
thus  that  we  see  all  objects  in  the  direction  of  the  rays 
which  reach  our  eyes. 

But  1  have  a  further  proof  in  favour  of  what  1  have 
advanced,  v\hich  I  hope  will  remove  your  remaining 
doubts  ;  I  shall,  however,  defer  it  till  our  next  meeting, 
a&  the  lesson  has  been  sufficiently  long  to-day. 


CONVERSATION  XV. 


OVTlCS—continued. 

ON    THE    ANGLE    OF    VISION,   AND    THE    REFLECTION    OF 
MIRRORS. 

AUTGLE   OF  VISION. REFLECTION   OF  PLAIN  MIRRORS. REFLECTION 

OF    CONVEX   MIRRORS. REFLECTION   OF   CONCAVE   MIRRORS. 


Caroline. 

Well,  Mrs.  "B.,  I  am  very  impatient  to  hear  what  fur- 
ther proofs  you  have  to  offer  in  support  of  your  theory. 
You  must  allow  that  it  was  rather  provoking  to  dismiss 
us  as  you  did  at  our  last  meeting. 

Mrs.  B,  You  press  so  hard  upon  me  with  your  ob- 
jections, that  you  must  give  me  time  to  recruit  my 
forces. 

Can  you  tell  me,  Caroline,  why  objects  at  a  distance 
appear  smaller  than  they  really  are  ^ 

Caroline.  I  know  no  other  reason  than  their  dis- 
tance. 

Mrs.  B.  I  do  not  think  I  have  more  cause  to  be  satis- 
fied with  your  reasons,  than  you  appear  to  be  with  mine^ 
y 


254  ON  THE  ANGLE  OF  VISION. 

We  must  refer  again  to  the  camera  obscum  to  account 
for  this  circumstance  and  you  will  find,  that  tliedifter- 
eiit  apparent  dimensions  of  objects  at  difterent  nlistan- 
ces,  proceed  from  our  seeing,  not  the  objects  themselves, 
but  merely  their  image  on  the  retina.  Fig.  1.  plate 
XVII.  represents  a  row  of  trees,  as  viewed  in  the  ca- 
mera obscura.  I  have  expressed  the  direction  of  the 
rays,  from  the  objects  to  the  image,  by  lines.  Now, 
observe,  the  ray  which  comes  from  the  top  of  the  near- 
est tree,  and  that  which  comes  from  the  foot  of  the  same 
tree,  meet  at  the  aperture,  forming  an  angle  of  about 
twenty-five  degreess  ;  this  is  called  the  angle  of  vision, 
under  which  we  see  the  tree.  These  rays  cross  each 
other  at  the  aperture,  forming  equal  angles  on  each  side 
of  it,  and  lepresent  the  tree  inverted  in  the  camera  ob- 
scura. The  degrees  of  the  image  are  considerably 
snialler  than  those  of  the  object,  but  the  proportions 
are  perfectly  preserved. 

Now  let  us  notice  the  upper  and  lower  ray,  from  the 
most  distant  tree  ;  they  form  an  angle  of  not  more  than 
twrlve  or  fifteen  degrees,  and  an  image  of  proportional 
dimensions.  Thus,  two  objects  of  the  same  size,  as 
the  two  trees  of  tlie  avenue,  form  figures  of  different 
sizes  in  the  camera  obscura,  according  to  their  dis- 
tance ;  or,  in  other  words,  according  to  the  angle  of  vi- 
sion under  which  they  are  seen.  Da  you  understand  this? 

Caroline.     Perfectly. 

Mrs\  B,  Then  you  have  only  to  suppose  that  the 
represe?itation  in  the  camera  obscura  is  similar  to  that 
on  the  retina. 

Now  since  objects  in  the  same  magnitudes  appear 
to  be  of  different  dimensions,  when  at  different  distan* 


ON  THE  ANGLE  OF  VISION.  255 

Ces  from  us,  let  me  ask  you,  which  it  is  that  we  see ; 
the  reai  objects,  which  we  know  do  not  vary  in  size,  or 
the  images,  which  we  know  do  vary  according  to  the 
angle  of  vision  under  which  we  see  them  ? 

Caroline.  I  must  confess  that  reason  is  in  favour 
of  the  latter.  But  does  that  chair  at  the  further  end  of 
the  room  form  an  image  on  my  retina  much  smaller 
than  this  which  is  close  to  me  ?  they  appear  exactly  of 
the  same  size. 

Mrs.  B.  I  assure  you  they  do  not.  The  experience 
we  acquire  by  the  sense  of -touch  corrects  the  errors 
of  our  sight  with  regard  to  objects  within  our  reach. 
You  are  so  perfectly  convinced  of  the  real  size  of  ob- 
jects which  you  can  handle,^  that  you  do  not  attend  to 
their  apparent  difference. 

Does  that  house  appear  to  you  much  smaller  than 
when  you  are  close  to  it  ? 

Caroline,    No,  because  it  is  very  near  us. 

Mrs.  B.  And  yet  you  can  see  the  whole  of  it  through 
one  of  the  windows  of  this  room.  The  image  of  the* 
house  on  your  retina  must,  therefore  be  smaller  tlian 
that  of  the  window  through  which  you  see  it.  It  is 
your  knowledge  of  the  real  size  of  the  house  which  pre- 
vents your  attending  to  its  apparent  magnitude.  If  you 
were  accustomed  to  draw  from  nature,  you  would  be 
fully  aware  of  this  difference. 

Emily.  And  pray,  what  is  the  reason  that,  when 
we  look  up  an  avenue,  the  trees  not  only  appear  small- 
er as  they  are  more  distant,  but  seem  gradually  to  ap- 
proach each  other  till  they  meet  in  a  point  ? 

Mrs.B.  Not  only  the  trees,  but  the  road  which  se- 
.parates  the  two  rows,  forms  a  smaller  visual  angle,  in 


256  ON  THE  ANGLE  OF  VISION. 

proportion  as  it  is  more  distant  from  us  ;  therefore  the 
width  of  the  road  gradually  diminishes  as  well  as  the 
size  of  the  trees,  till  at  length  the  road  apparently  ter- 
minates in  a  point,  at  which  the  trees  seem  to  meet. 

But  this  effect  of  the  angle  of  vision  will  be  more 
fully  illustrated  by  a  little  model  of  an  avenue,  which  I 
have  made  for  that  purpose.  It  consists  of  six  trees, 
leading  to  a  hexagonal  temple,  and  viewed  by  an  eye, 
on  the  retina  of  which  the  picture  of  the  objects  is  de- 
lineated. 

I  beg  that  you  will  not  criticise  the  proportions ;  for 
though  the  eye  is  represented  the  size  of  life,  while 
the  trees  are  not  more  than  three  inches  high,  the  dispro- 
portion does  not  aff^ect  the  principle,  which  the  model 
is  intended  to  elucidate. 

Emily,  The  threads  which  pass  from  the  objects 
through  the  pupil  of  the  eye  to  the  retina,  are,  I  sup- 
pose, to  represent  the  rays  of  light  which  convey  the 
image  of  the  objects  to  the  retina? 

Mrs.  B,  Yes.  1  have  been  obliged  to  limit  the  rays 
to  a  very  small  number,  in  order  to  avoid  confusion 
there  are,  you  see,  only  two  from  each  tree. 

Caroline,  But  as  one  is  from  the  summit,  and  the 
other  from  the  foot  of  the  tree,  they  exemplify  the  dif- 
ferent angles  under  which  we  see  objects  at  different 
distances,  better  than  if  there  were  more. 

Mrs,  JR.  There  are  seven  rays  proceeding  from  the 
temple,  one  from  the  summit,  and  two  from  each  of  the 
angles  that  are  visible  to  the  eye,  as  it  is  situated  ;  from 
these  you  may  form  a  just  idea  of  the  difference  of  the 
angle  of  vision  of  objects  viewed  obliquely,  or  in  front  ; 
for  though  the  six  sides  of  the  temple  are  of  equal  di- 


ON  THE  ANGLE  OF  VISION.  25/ 

mensions,  that  which  is  opposite  to  the  eye  is  seen  un- 
der a  much  iar<^er  angle,  than  those  which  are  viewed 
obliquely.  It  is  on  this  principh  that  the  laws  of  per- 
spective are  founded. 

Emilif.  1  am  \Qvy  glad  to  know  that,  for  I  have  late- 
ly be^un  to  learn  perspective,  which  appeared  to  me  a 
\e.vy  dry  study  ;  but  now^  that  I  am  acquainted  with 
the  principles  on  which  it  is  founded,  1  shall  find  it 
much  more  interesting. 

Caroline,  In  drawing  a  view  from  nature,  then,  we 
do  not  copy -the  real  objects,  but  the  image  they  form 
on  the  retinaof  our  eyes? 

Mrs,  B.  Certainly.  In  sculpture,  we  copy  nature 
as  she  really  exists  ;  in  ])ainting,  we  represent  her  as 
she'anpears  to  us.  It  Was  on  this  account  that  I  found 
it  difficult  to  explain  by  a  drawing  thii  effects  of  the 
angle  of  vision,  and  was  under  the  necessity  of  con- 
structing a  model  for  tliat  purpose. 

Emilij,  1  hope  you  will  allow  us  to  keep  this  model 
some  time,  in  order  to  study  it  more  com|)leteIy,  for  a 
great  deal  may  be  learned  from  it;  it  illustrates  the 
nature  of  the  angle  of  virion,  the  apparent  diminution 
of  distant  objects,  and  the  inversion  of  the  ima^e  on 
the  retina.  But  pray,  why  are  the  threads  that  repre- 
sent the  rays  of  light,  coloured,  the  same  as  the  objects 
from  which  they  proceed  ? 

Mrs,  B,  That  is  a  question  tvhich  you  must  excuse 
my  answeriug  at  present,  but  I  promise  to  explain  it  to 
you  in  due  time. 

J  consent  very  willing^ly  to  your  keeping  i\\Q  model, 
on  condition  that  you  will  make  an  imitation  of  it,  on 
the  same  principle,  but  representing  different  objects. 

Y    5 


258  ON  THE  ANGLE  OF  VISION. 

We  must  now  conclude  the  observations  that  remain 
to  be  made  on  the  angle  of  vision. 

If  an  object,  with  an  ordinary  degree  of  illumination, 
does  not  subtend  an  angle  of  more  than  two  seconds  of 
a  degree,  it  is  invisible.  There  are  consequently  two 
cases  in  which  objects  may  be  invisible,  either  if  they 
are  too  small,  or  so  distant  as  to  form  an  angle  less  than 
two  seconds  of  a  degree. 

In  like  manner,  if  the  velocity  of  a  body  does  not 
exceed  20  degrees  in  an  hour,  its  motion  is  impercep- 
tible. 

Caroline,  A  very  rapid  motion  may  then  be  imper- 
ceptible, provided  the  distance  of  the  moving  body  is 
sufficiently  great. 

Mrs.  B.  Undoubtedly  ',  for  the  greater  its  distance, 
the  smaller  will  be  the  angle  under  which  its  motion 
will  appear  to  the  eye.  It  is  for  this  reason  that  the 
motion  of  the  celestial  bodies  is  invisible,  notwithstand. 
ing  their  immense  velocity. 

Emily,  I  am  surprised  that  so  great  a  velocity  as 
20  degrees  an  hour  should  be  invisible. 

Mrs,  B,  The  real  velocity  depends  altogether  on 
the  space  comprehended  in  each  degree ;  and  this 
space  depends  on  the  distance  of  the  object,  and  the 
obliquity  of  its  path.  Observe,  likewise,  that  we  can- 
not judge  of  the  velocity  of  a  body  in  motion  unless 
we  know  its  distance  ;  for  supposing  two  men  to  set  oft* 
at  the  same  moment  from  A  and  B,  (fig.  2.)  to  walk  each 
to  the  end  of  their  respective  lines  C  and  D  ;  if  they 
perform  their  walk  in  the  same  space  of  time,  they  must 
have  proceeded  at  a  very  different  rate,  and  yet  to  an 
eye  situated  at  E,  they  will  appear  to  have  moved  with 


@N  THE  ANGLE  OF  VISION.  ,^9 

equal  velocity :  because  they  will  both  have  gone  through 
an  equal  number  of  degrees,  though  over  a  very  un- 
equal length  of  ground.  Sight  is  an  extremely  useful 
sense  no  doubts,  but  it  cannot  always  be  relied  on,  it  de- 
ceives us  both  in  regard  to  the  size  and  the  distance  of 
objects  ;  indeed  our  senses  would  be  very  liable  to  lead 
us  into  error,  if  experience  did  not  set  us  right. 

Emily,  Between  the  two,  I  think  that  we  contrive 
to  acquire  a  tolerably  accurate  idea  of  objects. 

Mrs,  B,  At  least  sufficiently  so  for  the  general  pur- 
poses of  life.  To  convince  you  how  requisite  expe- 
rience is  to  correct  the  errors  of  sight,  f  shall  relate  to 
you  the  case  of  a  young  man  who  was  blind  from  his 
infancy,  and  who  recovered  his  sight  at  the  age  of  four- 
teen, by  the  operation  of  couching.  At  first,  he  had  no 
idea  either  of  the  size  or  distance  of  objects,  but  ima- 
gined that  every  thing  he  saw  touched  his  eyes  ;  and  it 
was  not  till  after  having  repeatedly  felt  them,  and 
walked  from  one  object  to  another,  that  he  acquired  an 
idea  of  their  respective  dimensions,  their  relative  situ- 
ations, and  their  distances. 

Caroline,  The  idea  that  objects  touched  his  eyes,  is 
however  not  so  absurd,  as  it  at  first  appears  ;  for  if  we 
consider  that  we  see  only  the  image  of  objects,  this 
image  actually  touches  our  eyes. 

Mrs,  B,  That  is  doubtless  the  reason  of  the  opinion 
he  formed,  before  the  sense  of  touch  had  corrected  his 
judgment. 

Caroline,  But  since  an  image  must  be  formed  on 
the  retina  of  each  of  our  eyes,  why  do  we  not  see  ob- 
jects double  ? 

Mrs,  B,    The  action  of  the  rays  on  the  optic  nerve 


360  ON    THE  ANGI.E  OF  VrSION. 

of  each  eye  is  so  perfectly  similar,  that  they  produce  but 
a  single  sensation,  the  mind  therefore  receives  the  same 
idea,  from  the  retina  of  both  eyes,  and  conceives  the 
object  to  be  single. 

Caroline,  This  is  difficult  to  comprehend,  and! 
should  think,  can  be  but  conjectural. 

Mrs,  B,  I  can  easily  'convince  you,  that  you  have  a 
distinct  image  of  an  object  formed  on  the  retina  of  each 
eye.  Look  at  the  bell-rope,  aiid  tell  me  do  you  see  it 
to  the  right  or  the  left  of  the  pole  of  tlie  iire-skreen  ? 

Caroline,    A  little  to  the  right  of  it. 

Mrs.  B,  Then  shut  your  right  eye,  and  you  will  see 
it  to  the  l^ft  of  the  pole. 

Caroline,     That  is  true  indeed  ! 

•Mrs,  B,  There  are  evidently  two  representations 
of  the  bell-rope  in  different  situations,  which  must  be 
owing  to  an  image  of  it  being  formed  on  both  eyes  ;  if 
the  action  of  the  rays  tKeref<)re  on  each  retina  were  not 
so  perfectly  similar  as  to  produce  but  one  sensation,  we 
should  see  double,  and  we  find  that  to  be  the  case  with 
many  persons  who  are  afflicted  with  a  disease  in 
one  eye,  which  prevents  the  rays  of  light  from  affecting 
it,  in  the  same  manner  as  the  other, 

Em  lb/.  Pray,  Mrs.  B.,  when  we  see  the  image  of 
an  object  in  a  lookinu:-glass,  why  is  it  not  inverted  as  in 
the  camera  obscura,  and  on  the  retina  of  the  eye  ? 

Mrs,  B.  Because  the  rays  do  not  enter  the  mirror 
by  a  small  aperture,  and  cross  each  other,  as  they  do  at 
the  orifice  of  a  camera  obscura,  or  the  pupil  of  the  eye. 

When  vou  view  yourself  in  a  mirror,  the  rays  from 
your  eyes  fall  perpendicularly  upon  it,  and  are  re Hected 
in  the  same  line ;  the  image  is  therefore  described  be- 


Plate  xvn. 


Pnh.  hvj.\:fh„„,,h,;-v^-  t'l.il.i./:' 


REFLECTION  OF  MIRRORS.  261 

hind  the  glass,  and  is  situated  in  the  same  manner  as 
the  object  before  it. 

Emily.  Yes,  I  see  that  it  is;  but  the  looking-glass 
is  not  nearly  so  tall  as  1  am,  how  is  it  therefore  that  I 
can  see  the  whole  of  my  figure  in  it  ? 

Mrs.  B,  It  is  not  necessary  that  (he  mirror  should 
be  more  than  half  your  height,  in  order  that  you  may 
see  the  whole  of  your  person  in  it,  (fig.  3.)  The  ray  of 
light  C  D  from  your  eye,  which  falls  perpendicularly  on 
the  mirror  B  D,  will  be  reflected  back  in  the  same  line ; 
but  the  ray  from  your  feet  will  fall  obliquely  on  the 
mirror,  for  it  must  ascend  in  order  to  reach  it ;  it  will 
therefore  be  reflected  in  the  line  D  A :  and  since  we 
view  objects  in  the  direction  of  the  reflected  rays,  which 
reach  the  eye,  and  that  the  image  appears  at  the  same 
distance  behind  the  mirror  that  the  object  is  before  it, 
we  must  continue  the  line  A  D  to  E,  and  the  line  C  D 
to  F,  at  the  termination  of  which,  the  image  will  be  re- 
presented. 

Emily.  Then  I  do  not  understand  why  I  should  not 
see  the  whole  of  my  person  in  a  much  smaller  mirror, 
for  a  ray  of  light  from  my  feet  would  always  reach  it, 
though  more  obliquely. 

Mrs.  B.  True ;  but  the  more  obliquely  the  ray  falls 
on  the  mirror,  the  more  obliquely  it  will  be  reflected; 
the  ray  would  therefore  be  reflected  above  your  head, 
and  you  could  not  see  it.  This  is  shown  by  the  dotted 
line.  (fig.  3.) 

Now  stand  a  little  to  the  right  of  the  mirror,  so  that 
the  rays  of  light  from  your  figure  may  fall  obliquely 
en  it— 


262  REFLECTION  OF  MIRRORSi 

Emibj,  There  is  no  image  formed  of  me  in  the  glass 
now. 

Mv^,  B,  I  beg  your  pardon,  there  is  ;  but  you  can- 
not see  it,  because  the  incident  rajs  failing  obliquely  on 
the  mirror  will  be  reflected  obliquely  in  the  opposite  di-' 
rection,  tlie  angles  of  incidence  and  of  reflection  being 
equal.  Caroline,  place  yourself  in,  the  direction  of  the 
reflected  rays,  and  tell  me  whether  you  do  not  see 
Emily's  image  in  the  glass  ? 

Caroline,  Let  me  consider.>^-In  order  to  look  in  the 
direction  of  the  reflected  rays,  I  must  place  myself  as 
much  to  the  left  of  the  glass  as  Emily  stands  to  the 
right  of  it— Now  1  see  her  image,  but  it  is  not  straight 
before  me,  but  before  her  ;  and  appears  at  the  same  dis- 
tance behind  the  glass,  as  she  is  in  front  of  if. 

Mrs.  /?.  You  must  recollect,  that  we  always  see 
objects  in  the  direction  of  the  last  rays  which  reach  our 
eyes.  Figure  4.  represents  an  eye  looking  at  the  image 
of  a  vase,  reflected  by  a  mirror ;  it  must  see  it  in  the 
direction  of  the  ray  A  B,%s  that  is  the  ray  which  brings 
the  image  to  the  eye  :  prolong  ttie  ray  to  C,  and  in  that 
spot  will  the  image  appear. 

Caroline,  I  do  not  understand  why  a  looking-glass 
reflects  the  rays  of  light :  for  glass  is  a  transparent 
body  which  should  transmit  them  ? 

Mi^F,  B,  It  is  not  the  glass  that  reflects  the  rays 
which  form  i\^e  image  you  behold,  but  the  mercury  be- 
hind it.  The  glass  acts  chiefly  as  a  transparent  case, 
through  which  the  rays  find  an  easy  passage. 

Caroline.  Why  then  should  not  mirrors  be  made 
simply  of  mercury  ? 

Mrs.   B.     Because  mercury  is  a  fluid.     By  amalga- 


REFLECTION  OF  MIRRORS.  263 

mating  it  with  tin  foil,  it  becomes  ot  the  consistence  of 
paste,  attaches  itself  to  the  glasB,  antl  forms  in. fact  a 
mercurial  mirro.r,  which  would  be  much  more  perfect 
witiiout  its  glass  cover,  for  the  purest  glass  is  never 
perfectly  transparent ;  some  of  the  rajs  therefore  are 
lost  during  their  passage  through  it,  by  being  either  ab- 
sorbed, or  irregularly  reflected. 

This  imperfection  of  glass  mirrors  has  introduced  the 
use  of  metallic  mirrors,  for  optical  purposes. 

Emily,  But  since  all  opaque  bodies  reflect  the  rays 
of  light,  I  do  not  understand  why  they  are  not  all 
mirrors  ? 

Caroline.  A  curious  idea  indeed,  sister ;  if  would 
be  very  gratifying  to  see  oneself  in  every  object  at 
which  one  looked. 

Mrs.  B.  It  is  very  true  that  all  opaque  objects  re- 
flect light ;  but  the  surface  of  bodies  in  general  is  so 
rough  and  uneven,  that  their  reflection  is  extremely  ir- 
regular, which  prevents  the  rays  from  forming  an  image 
on  the  retina.  This  you  will  be  able  to  understand 
better,  when  I  shall  explain  to  you  the  nature  of  vision, 
and  the  structure  of  the  eye. 

You  may  easily  conceive  the  variety  of  directions  in 
which  rays  would  be  reflected  by  a  nutmeg-grater,  on 
account  of  the  inequality  of  its  surface,  and  the  num- 
ber of.  holes  with  which  it  is  pierced.  All  solid  bodies 
resemble  the  nutmeg-grater  in  these  respects,  more  Or 
less ;  and  it  is  only  those  which  are  susceptible  of  re- 
ceiving a  polish,  that  can  be  made  to  reflect  the  rays 
with  regularity.  As  hard  bodies  are  of  the  closest  tex- 
ture, the  least  porous,  and  capable  of  taking  the  highest 


264  REFLECTION  OF  CONVEX  MIRRORS. 

polish,  they  make  the  best  mirrors ;  none  therefore  are 
so  weJ!  calculated  for  this  purpose  as  metals. 

Caroline.  But  the  property  of  regular  reflection  is 
not  confined  to  this  class  of  bodies  ;  for  I  have  often 
seen  myself  in  a  highly  polished  mahogany  table. 

Mrs,  B.  Certainly ;  but  as  that  substance  is  less  du- 
rable, and  its  reflection  less  perfect,  than  that  of  metals, 
I  believe  it  would  seldom  be  chosen  for  the  purpose  of 
a  mirror. 

There  are  three  kinds  of  mirrors  used  in  optics  ;  the 
plain  or  flat,  vihich  are  the  common  mirrors  we  have 
just  mentioned  ;  convex  mirrors  ;  and  concave  mirrors. 
The  reflection  of  the  two  latter  is  very  different  from 
that  of  the  former.  The  plain  mirror,  we  have  seen, 
does  not  alter  the  direction  of  the  reflected  rays,  and 
forms  an  image  behind  the  glass  exactly  similar  to  the 
object  before  it.  A  convex  mirror  has  the  peculiar 
property  of  making  the  reflected  rays  diverge,  by 
which  means  it  diminishes  the  image ;  and  a  concave 
mirror  makes  the  rays  converge,  and  under  certain  cir- 
cumstances, magnifies  the  image. 

Emily.  We  have  a  convex  mirror  in  the  drawing- 
room,  which  forms  a  beautiful  miniature  picture  of  the 
objects  in  the  room  ;  and  I  have  often  amused  myself 
with  looking  at  my  magnified  face  in  a  concave  mirror. 
But  [  hope  you  will  explain  to  us  why  the  one  enlarges 
while  the  other  diminishes  the  objects  it  reflects. 

Mrs.  B.  Let  us  begin  by  examining  the  reflection 
of  a  convex  mirror.  This  is  formed  of  a  portion  of 
the  exterior  surface  of  a  sphere.  When  several  paral- 
lel rays  fall  upon  it,  that  ray  only  which,  if  prolonged, 
would  pass  through  the  centre  or  axis  of  the  mirror. 


Piih.hy  J.Y.Huritphj'eys  PhiLtd'f 


REFLECTION  OF  CONVEX  MIRRORS.  265 

is  perpendicular  to  it.  In  order  to  avoid  confusion  I  have, 
iii  li^.  1.  plate  XVlli.  drawn  only  three  paraUel  lines, 
A  B,  C  D,  E  F,  to  represent  rays  falling  on  the  con- 
vex mirror  M  N  ;  the  middle  ray,  you  will  observe,  is 
perpendicular  to  the  mirror,  the  others  fall  on  it  ob- 
liquely. 

Caroline,  As  the  three  rays  are  parallel,  why  are 
they  not  all  perpendicular  to  the  mirror  ? 

Mrs.  B.  They  would  be  so  to  a  flat  mirror  ;  but  as 
this  is  spherical,  no  ray  can  fall  perpendicularly  upon 
it  which  is  not  directed  towards  the  centre  of  the 
sphere. 

Emily,  Just  as  a  weight  falls  perpendicularly  to  the 
earth  when  gravity  attracts  it  towards  the  centre. 

Mrs,  B,  In  ordtr,  tlierefore,  that  rays  may  fall  per- 
pendicularly to  the  mirror  at  Band  P\  the  rays  must  be 
in  the  direction  of  the  dotted  lines,  which,  you  may  ob- 
serve, meet  at  the  centre  0  of  the  sphere,  of  which  the 
mirror  forms  a  portion. 

Now  can  you  tell  me  in  what  direction  the  three  rays 
A  B,  C  D,  E  F,  will  be  reflected  ? 

Emily.  Yes,  I  think  so :  the  middle  ray  failing  per- 
pendicularly on  the  mirror,  will  be  reflected  in  the  same 
line:  the  two  others  falling  obliquely,  will  be  reflected 
obliquely  to  G  H  ;  for  the  dotted  lines  you  have  drawn 
are  perpendiculars,  which  divide  their  angles  of  inci- 
dence and  reflection. 

Mrs,  B.  Extremely  well,  Emily  :  aqd  since  we  see 
objects  in  the  direction  of  the  reflected  ray,  we  shall 
see  the  image  at  L,  which  is  the  point  at  v\4iich  the  re- 
flected rays,  if  continued  through  the  mirror,  would 
unite  and  form  an  image.    This  point  is  equally  distant 


266  REFLECTION  O^  CONVEX  MIRRORS. 

from  the  surface  and  centre  of  the  sphere,  and  is  call- 
ed the  imaginary  focus  of p the  mirror. 

Caroline.    Praj,  what  is  the  meaning  of  focus  ? 

Mrs,  B.  A  point  at  which  converging  rays  unite. 
And  it  is  in  this  case  called  an  imaginary  focus  ;  be- 
cause the  rays  do  not  really  unite  at  that  point,  but  only 
appear  to  do  so:  for  the  rays  do  not  pass  through  the 
mirror,  since  they  are  reflected  by  it. 

Emily.  1  do  not  yet  understand  why  an  object  ap- 
pears smaller  when  viewed  in  a  convex  mirror. 

Mrs,  B,  It  is  owing  to  the  divergence  of  the  reflect- 
ed rays.  You  have  seen  that  a  convex  mirror  converts, 
by  reflection,  parallel  rays  into  divergent  rays  ;  rays 
that  fall  upon  the  mirror  divergent,  are  rendered  still 
more  so  by  reflection,  and  convergent  rays  are  reflected 
either  parallel,  or  less  convergent.  If  then  an  object 
be  placed  before  any  part  of  a  convex  mirror,  as  the 
vase  A  B,  fig.  2.  for  instance,  the  two  rays  from  its  ex- 
tremities, falling  convergent  on  the  mirror,  will  be  re- 
flected less  convergent,  and  will  not  come  to  a  focus 
till  they  arrive  at  C  ;  then  an  eye  placed  in  tlie  direc- 
tion of  the  reflected  rays,  will  see  the  image  foinied  in 
(or  rather  behind)  the  mirror  at  a  b, 

Caroline,  But  the  reflected  rays  do  not  appear  to 
me  to  converge  less  than  the  incident  rays.  I  should 
have  supposed  that,  on  the  contrary,  they  converged 
more,  since  they  meet  in  a  point  ? 

Mrs,  B,  They  would  unife  sooner  than  they  actual- 
ly do,  if  they  were  not  less  convergent  than  the  inci- 
dent rays :  for  observe,  that  if  the  incident  rays,  in- 
stead of  being  reflected  by  the  mirror,,  continued  their 
course  in  their  original  direction,  they  would  come  to  a 


REFLECTION  OP  CONCAVE  MIRRORS.  267 

tocus  at  D,  which  is  considerably  nearer  to  the  mirror 
than  at  C  ;  the  image  is  therefore  seen  under  a  smaller 
angle  than  the  object ;  and  the  more  distant  the  latter 
is  from  the  mirror,  the  less  is  the  image  reflected  by  it. 

You  will  now  easily  understand  the  nature  of  the  re- 
flection of  concave  mirrors.  These  are  formed  of  a 
portion  of  the  internal  siwface  of  a  hollow  sphere,  and 
their  peculiar  property  is  to  converge  the  rays  of  light. 

Can  you  discover,  Caroline,  in  what  direction  the 
three  parallel  rays,  A  B,  CD,  E  F,  which  fall  on  the 
concave  mirror  M  N  (fig.  3.)  are  reflected  ? 

Caroline.  I  believe  I  can.  The  middle  ray  is  sent 
back  in  the  same  line,  as  it  is  in  the  direction  of  the 
axis  of  the  mirror  ;*  and  the  two  others  will  be  reflected 
obliquely,  as  they  fall  obliquely  on  the  mirror.  I  must 
now  draw  two  dotted  lines  perpendicular  to  their  points 
of  incidence,  which  will  divide  their  angles  of  inci- 
dence and  reflection ;  and  in  order  that  those  angles 
may  be  equal,  the  two  oblique  rays  must  be  reflected  to 
L,  where  the}^  will  unite  with  the  middle  ray. 

,Mrs.  B,  Very  well  explained.  Thus  you  see,  that 
when  any  number  of  parallel  rays  fall  on  a  concave 
mirror,  they  are  all  reflected  to  a  focus  ;  for  in  propor- 
tion as  the  rays  are  more  distant  from  the  axis  of  the 
mirror,  they  fall  n»ore  obliquely  upon  it,  and  are  more 
obliquely  reflected  ;  in  consequence  of  which  they  come 
to  a  focus  in  the  direction  of  the  axis  of  the  mirror,  at 
a  point  equally  distant  from  the  centre  and  the  surface 
of  the  sphere,  and  this  point  is  not  an  imaginary  focus, 
as  happens  with  the  convex  mirror,  but  is  the  true  focus 
at  which  the  rays  unite. 


«68  REFLECTION  OF  CONCAVE  MlRtlORS. 

Emily,  Can  a  mirror  form  more  tlian  one  focus  by 
reflecting  rays  ? 

Mrs.  B,  Yes.  If  rajs  fall  convergent  on  a  concave 
mirror,  (fig.  4.)  they  are  sooner  brought  to  a  focus,  L, 
than  parallel  rajs:  their  focus  is  therefore  nearer  to 
the  mirror  M  N.  Divergent  rajs  are  brought  to  a  more 
distant  focus  than  parallel  rajs,  as  in  fig.  5,  where  the 
focus  is  at  L ;  but  the  true  focus  of  mirrors,  either  con^ 
vex  or  concave,  is  that  of  parallel  rajs,  which  is  equal Ij 
distant  from  the  centre,  and  the  surfat  e  of  the  sphere. 

I  shall  now  show  jou  the  reflection  t)f  real  rajs  of 
light,  by  a  metallic  concave  mirror.  This  is  one  made 
of  polished  tin,  which  I  expose  to  the  sun,  and  as  it 
shines  bright,  we  shall  be  able  to  collect  the  rajs  into  a 
verj  brilliant  focus.  1  hold  a  piece  of  paper  where  I 
imagine  the  focus  to  be  situated  ;  jou  maj  see  bj  the 
vivid  spot  of  light  on  the  paper,  how  much  the  rajs 
converge:  but  it  is  not  yd  exactlj  in  the  focus ;  as  I 
approach  the  paper  to  that  point,  observe  how  the  bright- 
Bess  of  the  spot  of  light  increases,  while  its  size  di- 
minishes. 

Caroline,  That  must  be  occasioned  bj  the  rajs  be- 
coming closer  together.  I  think  jou  hold  the  paper 
just  in  the  focus  now,  the  light  is  so  small  and  dazzling 
— Oh,  Mrs.  B.,  the  paper  has  taken  fire  ! 

Mrs.  B,  The  rajs  of  light  cannot  be  concentrated, 
without,  at  the  same  time,  accumulating  a  proportional 
quantitj  of  heat :  hence  concave  mirrors  have  obtained 
the  name  of  burning-mirrors. 

Emily.  I  have  often  heard  of  the  surprising  effects 
of  buriiing-mirrors,  and  I  am  quite  delighted  to  under- 
stand their  nature. 


THE  KEFLECTION  OF  MIRRORS,  269 


Caroline,  It  cannot  be  the  true  focus  of  the  mirror  at 
which  the  rays  of  the  sun  unite,  for  as  they  proceed 
from  a  point,  they  must  fall  divergent  upon  the  mirror. 

Mrs,  B,  Strictly  speaking,  they  certainly  do.  But 
when  rays  come  from  such  an  immense  distance  as  the 
sun,  their  divergence  is  so  trilling,  as  to  be  impercep- 
tible ;  and  they  may  be  considered  as  parallel :  their 
point  of  union  is,  therefore,  the  true  focus  of  the  mir- 
ror, and  there  the  image  of  the  object  is  represented. 

Now  that  I  have  removed  the  mirror  out  of  the  in- 
fluence of  the  sun's  rays,  if  I  place  a  burning  taper  in 
the  focus,  how  will  its  light  be  reflected  ?  ([ig.  6.) 

Caroline,     That,  I  confess,  I  cannot  say, 

Mrs,  B,  The  ray  wiiich  falls  in  the  direction  of  the 
axis  of  the  mirror,  is  reflected  back  in  the  same  line  ;  but 
let  us  draw  two  other  rays  from  the  focus,  falling  on 
the  mirror  at  B  and  F ;  the  dotted  lines  are  perpendi- 
cular to  those  points,  and  the  two  rays  will  therefore 
be  reflected  to  A  and  E. 

Caroline,  Oh,  now  1  understand  it  clearly.  The 
rays  which  proceed  from  a  light  placed  in  the  focus  of 
a  concave  mirror  fall  divergent  upon  it,  and  are  reflect- 
ed parallel.  It  is  exactly  the  reverse  of  the  former 
experiment,  in  which  the  sun's  rays  fell  parallel  on  the 
mirrof,  and  were  reflected  to  a  focus. 

Mrs,  B,  Yes :  when  the  incident  rays  are  parallel, 
the  reflected  rays,  converge  to  a  focus  ;  when  on  the 
contrary,  the  incident  rays  proceed  from  the  focus, 
they  are  reflected  parallel.  This  is  an  important  law 
of  optics,  and  since  you  are  now  acquainted  with  the 
principles  on  which  it  js  founded,  I  hope  that  you  will 
iiot  forget  it. 

z  2 


270  THE  REFLECTION  OF  MIRRORS. 

Caroline.  I  am  sure  that  we  shall  not.  But,  Mrs. 
B.,  jou  said  that  the  image  was  formed  in  the  focus  of  a 
concave  mirror;  yet  I  have  freauentiv  seen  glas«  con- 
cave mirrors,  wnere  the  object  has  been  represented 
within  the  mirror,  in  the  same  manner  as  in  a  convex 
mirror. 

Mrs.  B,  That  is  the  case  only,  when  the  object  is 
placed  between  the  mirror  and  its  focus ;  the  image 
then  appears  magnified  behind,  or,  as  you  call  it,  with- 
in the  mirror. 

Caroline.  T  do  not  understand  why  the  image  should 
be  larger  than  the  object. 

Mrs.  B.  It  proceeds  from  the  convergent  property 
of  the  concave  mirror.  If  an  object,  A  B,  (fig.  7.)  be 
placed  between  the  mirror  ;ind  its  focus,  the  rajs  from 
its  extremities  fall  divergent  on  the  mirror,  and  on  be- 
ing reflected,  becomes  less  divergent,  as  if  they  pro- 
ceeded from  C  :  to  an  eye  placed  in  that  situation  the 
image  will  appear  magnified  behind  the  mirror  at  a  ft, 
since  it  is  seen  under  a  larger  angle  than  the  object. 

You  now,  I  hope,  understand  the  reflection  of  light 
by  opaque  bodies.  At  our  next  meeting,  we  shall  en- 
ter upon  another  property  of  light  no  less  interesting, 
which  is  called  refraction. 


CONVERSATION  XVI. 


ON  REFRACTION  AND  COLOURS. 

TRAXSMISSIO??     OF     LIGHT     BY    TRANSPARENT    BODIES. REFRAC- 
TION.  REFRACTTO-V    OF    THE    ATMOSPHERE. REFRACTION    OF    A 

LENS. REFRACTION     OF    THE     PRISM. OF     THE    COLOURS     OF 

RATS    OF    LIGHT OF    THE    COLOURS    OF    BODIES. 


Mrs.  B. 

The  refraction  of  light  will  furnish  the  subject  of  to- 
day's lesson. 

Caroline,  That  is  a  property  of  which  I  have  not  the 
faintest  idea. 

Mrs,  B,  It  is  the  effect  which  transparent  mediums 
produce  on  light  in  its  passage  through  them.  Opaque 
bodies  you  know^,  reflect  the  rays,  and  transparent  bo- 
dies transmit  them  ;  but  it  is  found,  that  if  a  ray,  in 
passing  from  one  medium  into  another  of  different  den- 
sity, fall  obliquely,  it  is  turned  out  of  its  course. 

Caroline.  It  must  then  be  acted  on  by  some  new 
power,  otherwise  it  would  not  deviate  from  its  first  di- 
rection. 


372  THE  REFRACTION  OF  LIGHT. 

Mrs,  B,  The  power  wnich  causes  the  deviation  of 
the  raj  appears  to  be  the  attraction  of  the  denser  medi- 
um. Let  us  suppose  the  two  mediums  to  be  air  and 
water;  if  a  ray  of  light  passes  from  air  into  water  it  is 
more  strongly  attracted  by  the  latter  on  account  of  its 
superior  density. 

Emily,  In  what  direction  does  the  water  attract 
the  ray  ? 

Mrs.  B,  It  must  attract  it  perpendicularly  towards 
it,  in  the  same  manner  as  gravity  acts  on  bodies. 

If  then  a  ray  A  B,  (fig.  1.  plate  XIX.)  fall  perpen- 
dicularly on  water,  the  attraction  of  the  water  acts  in 
the  same  direction  as  the  course  of  the  ray  :  it  will  not 
therefore  cause  a  deviation,  and  the  ray  will  proceed 
straight  on  to  E.  But  if  it  fall  obliquely,  as  the  ray  C 
B,  the  water  will  attract  it  out  of  its  course.  Let  i\& 
suppose  the  ray  to  have  approached  the  surface  of  a 
denser  medium,  and  that  it  there  begins  to  be  affected 
by  its  attraction  ;  this  attraction,  if  not  counteracted  by 
some  other  power,  would  draw  it  perpendicularly  to 
the  water,  at  B  ;  but  it  is  also  impelled  by  its  projec- 
tile force,  which  the  attraction  of  the  denser  medium 
cannot  overcome  ;  the  ray  therefore,  acted  on  by  both 
these  powers,  moves  in  a  direction  between  them,  and 
instead  of  pursuing  its  original  course  to  D,  or  being 
implicitly  guided  by  the  water  to  E,  proceeds  towards 
F,  so  that  the  ray  appears  bent  or  brok.en. 

Caroline.  I  understand  that  very  well  ;  and  is  not 
this  the  reason  that  oars  appear  bent  in  water  ? 

Mrs.  B,  It  is  owing  to  the  refraction  of  the  rays 
reflected  by  the  oar;  but  this  is  in  passing  from  a  d(?nse 


Plate  XK. 


Kc),i. 


}-'u,.:>. 


':'/       -  ■        N^  ■    ■    :■:■:';■:■;'•;•:; 


Fi,;  .  J . 


F,a>.  Try  J.Yimaitplnrysinnlart'} 


THE  REFRACTION  OF  LIGHT,  273 

to  a  rare  niediuni,  for  yon  kn?>vv  that  the  rays,  by  means 
of  vvliich  you  see  the  oar,  jmss  from  water  into  air. 

Emily.  But  I  do  not  understand  why  a  refraction 
takes  place  when  a  ray  passes  from  a  dense  into  a  rare 
medium  ;  1  should  suppose  that  it  wquld  be  rather  less; 
than  more  attracted  by  the  latter. 

Mrs,  B,  And  it  is  precisely  on  that  account  that 
the  ray  is  refracted.  C  B,fig.  2.  represents  a  ray  passing 
obliquely  from  glass  into  water  glass,  being  the  denser 
medium,  the  ray  will  be  more  strongly  attracted  by  that 
which  it  leaves  than  by  that  which  it  enters.  The  attrac- 
tion of  the  glass  acts  in  the  direction  A  B,  while  the  im* 
pulse  of  projection  would  carry  the  ray  to  F  ;  it  moves 
therefore  between  these  directions  towards  D. 

Emily.  So  that  a  contrary  refi  action  takes  place  when 
a  ray  passes  from  a  dense  into  a  rare  medium. 

Caroline.  But  does  not  the  attratcion  of  the  den- 
ser medium  affect  the  ray  before  it  touches  it  ? 

Mrs,  B.  The  distance  at  which  the  attraction  of  the 
denser  medium  acts  upon  a  ray  is  so  small  as  to  be  in- 
sensible ;  it  appears  therefore  to  be  refracted  only  at  the 
point  at  which  it  passes  from  one  medium  to  the  other. 

Now  that  you  understand  the  principle  of  refraction, 
I  will  show  you  the  refraction  of  a  real  ray  of  light.  Do 
you  see  the  flower  painted  at  the  bottom  of  the  inside 
of  this  tea-cup  ?  (Fig.  3.) 

Emily.  Yes.  But  now  you  have  moved  it  just  out 
of  sight,  the  rim  of  the  cup  hides  it. 

Mrs.  B.  Do  not  stir.  1  will  fill  the  cup  with  water, 
and  you  will  see  the  flower  again. 

Emily.  I  do  indeed!  Let  me  try  to  explain  this  : 
when  you  drew  the  cup  from  me  so  as  to  conceal  the 


274  THE  REFRACTION  OF  LIGHT. 

flo'ver,  the  rays  reflected  by  it  no  longer  met  my  eyes, 
but  were  directed  above  them  ;  but  now  that  you  have 
filled  the  cup  with  water,  they  are  refracted  by  the  at- 
traction of  the  water,  and  bent  downwards  so  as  again 
to  enter  my  eyes. 

Mrs.  B,  You  have  explained  it  perfectly:  Fig.  5. 
will  help  to  imprint  it  on  your  memory.  You  must  ob- 
serve that  when  the  flower  becomes  visible  by  the  re- 
fraction of  the  ray,  you  do  not  see  it  in  the  situation 
which  it  really  occupies,  but  an  image  of  the  flower 
hiiiher  in  the  cup;  for  as  objects  always  appear  to  be 
situated  in  the  direction  of  the  rays  which  enter  the  eye 
the  flower  will  be  seen  in  the  direction  of  the  reflected 
ray  at  B. 

Emily,  Then,  when  we  see  the  bottom  of  a  clear 
stream  of  water,  the  rays  which  it  reflects  being  refrac- 
ted in  their  passage  from  the  water  into  the  air,  will 
make  the  bottom  appear  higher  than  it  really  is. 

Mrs,  B,  And  the  water  will  consequently  appear 
more  shallow.  Accidents  have  frequently  been  occa- 
sioned by  this  circumstance  ;  and  boys  who  are  in  the 
habit  of  bathing  should  be  cautioned  not  to  trust  to 
the  apparent  shallowness  of  water,  as  it  will  always 
prove  deeper  than  it  appears  ;  unless,  indeed,  they  view 
it  from  a  boat  on  the  water,  which  will  enable  them  to 
look  perpendicularly  upon  it;  when  the  rays  from  the 
bottom  passing  perpendicularly,  no  refraction  will  take 
place. 

The  refraction  of  light  prevents  our  seeing  the  hea- 
venly bodies  in  their  real  situation  :  the  light  they  send 
to  U8  being  refracted  in  passing  into  the  atmosphere, 
we  see  the  sun   and  stars  in  the  direction  of  the  re- 


THE  REFRACTION  OF  LIGHT.  275 

fracted  ray  ;  as  described  in  fig.  4.  plate  XIX.,  the  d(3t- 
ted  line  represents  the  extent  of  the  atmosphere,  above 
a  po  non  of  the  earth,  E  B  E  :  a  ray  of  lij^ht  coming  from 
the  sun  S  falls  obliquely  on  it,  at  A,  ami  is  refracted  to 
B  ;  then,  since  we  see  the  object  in  the  direction  of  the 
refracted  ray,  a  spectator  at  B  will  see  an  image  of  the 
sun  at  C,  instead  of  the  real  object  at  S. 

Emily.  But  if  the  sun  were  immediately  over  our 
heads,  its  rays  falling  perpendicularly  on  the  atmosphere 
would  not  be  refracted,  and  we  should  then  see  the 
real  sun,  in  its  true  situation. 

Mrs.  B,  You  must  recollect  that  the  sun  is  vertical 
only  to  the  inhabitants  of  the  torrid  zone ;  its  rays, 
therefore,  are  always  refracted  in  these  climates.  There 
is  also  another  obstacle  to  our  seeing  the  heavenly  bodies 
in  their  real  situation  :  light,  though  it  moves  with  ex- 
treme velocity,  is  about  eight  minutes  and  an  half  in  its 
passage  from  the  sun  to  the  earth  ;  tl»eiefore,  when  the 
rays  reach  us,  the  sun  must  have  quitted  the  spot  he 
occupied  on  their  departure  ;  yet  we  see  him  in  the  di- 
rection of  those  rays,  and  consquently  in  a  situation 
which  he  had  abandoned  eight  minutes  and  an  half 
before. 

Emibj.  When  you  speak  of  the  sun's  motion,  you 
mean,  I  suppose,  his  apparent  motion,  produced  by  the 
diurnal  motion  of  the  earth  ? 

Mrs.  B.  No  doubt ;  the  effect  being  the  same, 
whether  it  is  our  earth,  or  the  heavenly  bodies  which 
m  )^e:  it  is  moie  easy  to  represent  things  as  they  ap- 
pear to  be,  than  as  they  really  are. 

Carolin-e.  During  the  morning,  then,  when  the  sun 
is  rising  towards  the  meridian,  we  must  (from  the  length 


276  THE  REFRACTION  OF  LIGHT. 

of  time  the  lii^ht  is  in  re:tching  us)  see  an  image  of  tlie 
sun  below  that  spot  which  it  really  occupies. 

Emily,  But  the  refi-action  of  the  atmosphere  coun- 
toracting  this  elfect,  we  may  perhaps,  between  the  two, 
set  the  sun  in  its  real  situation. 

Caroline,  And  in  the  afternoon,  when  the  sun  is 
sinking  in  the  west,  refraction  and  the  length  of  time 
which  the  light  is  in  reaching  the  earth,  will  conspire  to 
render  the  image  of  the  sun  higher  that  it  reallj  is. 

Mrs,  B,  The  refraction  of  the  sun's  rajs  bj  the  at- 
mosphere prolongs  our  days,  as  it  occasions  our  seeing  an 
image  of  the  sun  bofh  before  he  lises  and  after  he  sets^ 
for  below  the  horizon,  he  still  shines  upon  the  atn.os- 
phere,  and  hiij  rays  are  thence  refr^icted  to  the  earth. 
So  Iikewi3e  we  see  an  image  of  tlie  sun  before  he  rises, 
the  rays  that  previously  fall  upon  the  atmosphere  be- 
ing reilecied  to  the  earth. 

Car(jiine,  On  the  other  hand,  we  must  recollect  that 
light  \<  eight  minutes  and  an  half  on  ifs  journey  ;  so 
that,  by  the  time  it  reaches  the  earth,  the  sun  may  per- 
haps be  risen  above  the  horizon. 

Emihj,     Pray  do  not  glass  windows  refract  tlie  light  ? 

JlJrs,  B,  They  do;  but  this  refiaction  is  n(»t  per- 
ceptible, because,  in  passing  through  a  pane  of  glass  the 
rays  siifl'er  two  refractions,  which  being  in  contrary  di- 
rections, produce  the  s.ime  etfect  as  if  no  refraction  had 
taken  place. 

Mmibj,     I  do  not  understand  that. 

Mvfi,  B,  Fig.  D.  plate  XFX.  will  make  it  clear  to  you  :. 
A  A  represents  a  thick  pane  of  glass  seen  edgeways. 
Wh.en  the  ra\  B  approaches  the  glass  at  C,  it  is  refract- 
edbyit;  and  instead  of  continuing  its  course  in  the 


XHB  REFRACTION  OF  LIGHT.  277 

same  direction,  as  the  dotted  line  describes,  it  passes 
through  the  pane  to  D  ;  at  that  point  returning  into  ^he 
air^  it  is  again  refracted  by  the  glass,  but  in  a  contrary- 
direction  to  the  first  refraction,  and  in  consequence 
proceeds  to  E.  Now  you  must  observe  that  the  ray 
B  C  and  the  ray  D  E  being  parallel,  the  light  does  not 
appear  to  have  suftered  any  refraction. 

Emily.  So  that  the  effect  which  takes  place  on  the 
ray  entering  the  glass,  is  undone  on  its  quitting  it.  Or, 
to  express  myself  more  scientifically,  when  a  ray  of 
ligiit  passes  from  one  medium  into  anotlier,  and  through 
that  into  the  first  again,  the  two  refractions  being  equal 
and  in  opposite  directions,  no  sensible  eff*ect  is  pro- 
duced. 

Mrs.  B.  This  is  the  case  when  the  two  surfaces  of 
the  refracting  medium  are  parallel  to  each  other;  if 
they  are  not,  the  two  refractions  may  be  made  in  the 
same  direction,  as  I  shall  show  you. 

When  parallel  rays  (fij;.  6.)  tall  on  a  piece  f^f  ji^lass 
having  a  double  convex  surface,  and  which  is  called  a 
Lens,  that  only  wiiich  falls  in  the  direction  of  the  axis 
of  the  lens  is  perpend iculir  to  the  suriace  ;  the  other 
rays  falling  obliquely  a.e  refracted  towards  t\\^.  axis, 
and  will  meet  at  a  point  beyond  tlie  lens,  called  its 
focus. 

Of  the  three  rays,  A  B  C,  which  fall  on  the  lens  DE, 
the  rays  A  and  C  are  refracted  in  their  passage  through 
it,  to  a,  and  c,  and  on  quitting  the  lens  they  undergo  a 
second  refraction  in  the  same  direction  which  unites 
theai  with  tue  ray  B,  at  the  focus  F. 

Emily.     And  what  is  the  distance  of  tlve  focus  from 
the  surface  of  the  lens? 
A  a 


27S  THE  REFRACTION  OF  LIGHT. 

Mrs.  B,  The  focal  distance  depends  both  upon  the 
form  of  the  lens,  and  of  the  refractive  power  of  the 
substance  of  which  it  is  made  :  in  a  glass  lens,  both 
sides  of  which  are  equally  convex,  the  focus  is  situated 
nearly  at  the  centre  of  the  sphere  of  which  the  surface 
of  the  lens  forms  a  portion  ;  it  is  at  the  distance,  there- 
fore, of  the  radius  of  the  sphere. 

There  are  lenses  of  various  forms,  as  you  will  find 
described  in  fig.  1.  plate  XX.  The  property  of  those 
which  have  a  convex  surface  is  to  collect  the  rays  of 
light  to  a  focus  ;  and  of  those  which  have  a  concave 
surface,  on  the  contrary,  to  disperse  them.  For  the  rays 
A  C  falling  on  the  concave  lens  X  Y,  (fig.  7.  plate  XIX.) 
instead  of  converging  towards  the  ray  B,  which  falls 
on  the  axis  of  the  lens,  will  each  be  attracted  towards 
the  thick  edges  of  the  lens,  both  on  entering  and  quit- 
ting it,  and  will,  therefore,  by  the  first  refraction,  be 
made  to  diverge  to  a,  c,  and  by  the  second  to  rf,  e. 

Caroline.  And  lenses  which  have  one  side  flat  and 
the  other  convex  or  concave,  as  A  and  B,  fig.  1.  plate 
XX.,  are,  1  suppose,  less  powerful  in  their  refractions  ? 

Mrs.  B.  Yi^s;  they  are  called  plano-convex,  and 
plano-concave  lenses  :  the  focus  of  the  former  is  at  the 
distance  of  the  diameter  of  a  sphere,  of  which  the  con- 
vex surface  of  the  lens  forms  a  portion  ;  as  represented 
in  fig.  2.  plate 'XX.  The  three  parallel  rays,  A  B  C, 
aie  brought  to  a  focus  by  the  plano-convex  lens,  X  Y 
at  F. 

1  must  now  explain  to  you  the  refraction  of  a  trian- 
gular piece  of  glass,  called  a  prism.     (Fig.  3.) 

Emily.  The  three  sides  of  this  glass  are  flat ;  it 
cannot  therefore  bring  the  rays  to  a  focus  ;  nor  do  I  sup- 


Tnh.  hv  .l.Y.Hn^ip7ir^-VJ  Vllihi^V^ 


ON  REFRACTION  AND  COLOURS.      ^^ 

pose  that  its  refraction  will  be  similar  to  that  of  a  flat 
pane  of  glass,  because  it  has  not  two  sides  parallel ;  I 
cannot  therefore  conjecture  what  effect  the  refraction 
of  a  prism  can  produce. 

Jf/r.<?.  B,  The  refractions  of  the  light,  on  entering  and 
on  quitting  the  prism,  are  both  in  the  same  direction. 
(Fi;?:.  3.)  On  entering  the  prism  P,  the  ray  A  is  re- 
fracted from  B  to  C,  and  on  quitting  it  from  C  to  D. 

I  will  show  you  this  in  nature ;  but  for  this  purpose 
it  will  be  advisable  to  close  the  window-shutters,  and 
admit,  through  the  small  aperture,  a  ray  of  light,  which 
I  shall  refract  by  means  of  this  prism. 

Caroline,  Oh,  what  beautiful  colours  are  represent- 
ed on  the  opposite  wall !  There  are  all  the  colours  of 
the  rainbow,  and  with  a  brightness  I  never  saw  equalled. 
(Fig.  4.  plate  XX.) 

Emily,  I  have  seen  an  effect,  in  some  respect  simi- 
lar to  this,  produced  by  the  rays  of  the  sun  shining  upon 
glass  lustres ;  but  how  is  it  possible  that  a  piece  of 
\^hite  glass  can  produce  such  a  variety  of  brilliant 
colours  ? 

Mrs,  B,  The  colours  are  not  formed  by  the  prism, 
but  existed  in  the  ray  previous  to  its  refraction. 

Caroline,  Yet,  before  its  refraction,  it  appeared  per- 
fectly white. 

Mrs,  B,  The  white  rays  of  the  sun  are  composed  of 
coloured  rays,  which,  when  blended  together,  appear 
colourless  or  white. 

Sir  Isaac  Newton,  to  whom  we  are  indebted  for  the 
most  important  discoveries  respecting  light  and  colours, 
was  the  first  who  divided  a  white  ray  of  light,  and  found 
it  to  consist  of  an  assemblage  of  coloured  rays,  which 


280       ON  REFRAeTlGN  AND  GOLOURS. 

formed  an  image  upon  the  wall,  such  as  3'ou  now  see  ex- 
hibited, (fig.  4,)  in  which  are  displayed  the  following  se- 
ries of  colours  :  red^  orange,  yellow,  green,  blue,  in- 
digo, and  violet. 

Emily,  But  how  does  a  prism  separate  these  co- 
loured rays? 

Mrs,  IL  By  refraction.  It  appears  that  the  co- 
louied  rays  have  different  degrees  of  refrangibility  ;  in 
passing  through  the  prism,  therefore,  they  take  differ- 
ent directions  according  to  their  susceptibility  of  re- 
fraction. The  violate  rays  deviate  most  from  their  ori- 
ginal course;  tlVey  appear  at  one  of  the  ends  of  the  spec- 
trum A  B:  contiguous  to  the  violet,  are  the  blue  rays, 
be*' I  those  which  have  somewhat  less  refrangibility; 
tl  e  foijow,  in  succession,  the  green,  yellow,  orange, 
and,  lastly,  the  red,  which  are  the  least  refrangible  of 
the  coloured  rays. 

Caroline,  I  cannot  conceive  how  these  colours,  mixed 
together,  can  become  white  ? 

Mrs.  B.  That  I  cannot  pretend  to  explain  ;  but  it 
is  a  fact  that  the  union  of  these  colours,  in  the  propor- 
tions in  which  thev  appear  in  the  spectrum,  produce  in 
us  the  idea  of  whiteness.  If  you  paint  a  card  in  com- 
partjuents  with  these  seven  colours,  and  whirl  it  rapidly 
on  a  j)iH,  it  will  appear  white. 

But  a  more  decisive  proof  of  the  composition  of  a 
white  ray  is  afforded  by  reuniting  these  coloured  rays, 
and  forming  with  them  a  ray  of  white  light. 

Caroline.  If  you  can  take  a  ray  of  white  light  to 
pieces,  and  put  it  together  again,  I  shall  be  quite  sa- 
tisfied. 

Mrs.  B.    This  can  be  done  by  letting  the  coloured 


ON  REFRACTION  AND  COLOURS. 

fays,  wliich  have  been  separated  by  a  prism,  fall  upon  a 
lens,  which  will  converge  them  to  a  focus ;  and  if,  when 
thus  reunited,  we  find  that  they  appear  white  as  they 
did  before  refraction,  I  hope  that  you  will  be  convinced 
that  the  white  rays  are  a  compound  of  the  several  co- 
loured rays.  The  prism  P,  you  see,  (fig.  5.)  separates 
a  ray  of  white  light  into  seven  coloured  rays,  and  the 
lens  L  L  brings  them  to  a  focus  at  F,  where  they  again 
appear  white. 

Caroline.  You  succeed  to  perfection  :  this  is  indeed 
a  most  interesting  and  conclusive  experiment. 

Emily,  Yet,  Mrs.  B.,  I  cannot  help  thinking,  that 
there  may  perhaps  be  but  three  distinct  colours  in  the 
spectrum,  red,  yellow,  and  blue;  and  that  the  four 
others  may  consist  of  two  of  these  colours  blended  to- 
gether;  for,  in  pain ti no;,  we  find  that  by  mixing  red 
and  yellow,  we  produce  orange  ;  with  different  propor^- 
tionsof  red  and  blue,  we  make  violet  or  any  shade  of 
purple;  and  yellow  and  blue  form  green.  Now  it  is 
very  natural  to  suppose,  that  the  refraction  of  a  prism 
may  not  be  so  perfect  as  to  separate  the  coloured  rays 
of  light  completely,  and  that  those  which  are  contigu- 
ous in  order  of  refrangibility  may  encroach  on  each 
other,  and  by  mixing  produce  the  intermediate  colours, 
orange,  green,  violet,  and  indigo. 

Mrs.  B.  Your  observation  is,  I  believe,  neither 
quite  wrong,  nor  quite  right.  Dr.  VVollaston,  who  has 
refracted  light  in  a  more  accurate  manner  than  had  been 
previously  done,  by  receiving  a  very  narrow  line  of 
light  on  a  prism,  found  that  it  formed  a  spectrum,  con- 
sisting of  rays  of  four  colours  only ;  but  they  were  not 
exactly  those  you  have  named  as  primitive  colours,  iox 
Aa2 


582  ON  REFRACTION  AND  COLOURS. 

they  consisted  of  red,  green,  blue,  and  violet.  A  very 
narrow  line  of  yellow  was  visible,  at  the  limit  of  the  red 
and  wreen,  which  Dr.  Wollaston  attributed  to  the  over- 
lapping of  the  edges  of  the  red  and  green  light. 

Caroline,  But  red  and  green  mixed  together,  do 
not  produce  yellow.? 

Mrs,  B,  Not  in  painting;  but  it  may  be  so  in  the 
primitive  rays  of  the  spectrum.  Dr.  Wolkston  observed 
that,  by  increasing  the  breadth  of  the  aperture  by  which 
the  line  of  light  was  admitted,  the  space  occupied  by 
each  coloured  ray  in  the  spectrum  was  augmented,  in 
proportion  as  each  portion  encroached  on  the  neighbour- 
ing colour  and  mixed  with  it ;  so  that  the  intervention  of 
orange  and  yellow,  between  the  red  and  green,  is  owing 
he  supposes,  to  the  mixture  of  these  two  colours,  and 
the  blue  is  blended  on  the  one  side  with  the  green,  and 
on  the  other  with  the  violet,  forming  the  spectrum  as  it 
was  originally  observed  by  Sir  Isaac  Newton,  and  \\\\\q\\ 
I  have  just  shown  you. 

The  rainbow,  which  exliibits  a  series  of  colours  so 
analogous  to  those  of  the  spcctrnm,  is  formed  by  the  re- 
fraction of  the  sun's  rays  in  their  passage  through  a 
shower  of  lain,  eveiy  drop  of  which  acts  as  a  prism,  in 
separating  the  coloured  rays  as  they  pass  through  it. 

Enp.hj,  Pray,  Mrs.  B.,  cannot  the  sun's  rays  be  col- 
lected to  a  focus  by  a  lens  in  the  same  manner  as  they 
are  by  a  concave  mirror  ? 

Mrs,  B,  No  doubt  the  same  eflfect  is  produced  by 
the  refraction  of  a  lens  as  by  the  reflection  of  a  con- 
cave mirror:  in  the  first,  the  rays  pass  through  the  glass 
and  converge  to  a  focus  behind  it ;  in  the  latter,  they  are 
reflected  from  the  mirror,  and  brought  to  a  focus  before 


ON  REFRACTION  AND  COLOURS.       283 

it.  A  lens,  when  used  for  the  purpose  of  collecting  the 
sun's  rays,  is  called  a  burning  glass.  The  sun  now 
shines  very  bright ;  if  we  let  the  rajs  fall  pn  this  lens 
you  will  perceive  the  focus. 

Emity,  Oh  yes  :  the  point  of  union  of  the  rays  is 
very  luminous.  1  will  hold  a  piece  of  paper  in  the  focus, 
and  see  if  it  will  take  fire.  The  spot  of  light  is  extreme- 
ly brilliant,  but  the  paper  does  not  burn  ? 

Mrs.  B.  Try  a  piece  of  brown  paper; — that  you  see 
takes  fire  almost  immediately. 

Caroline.  This  is  surprising;  for  the  light  appear- 
ed to  shine  more  inter,  ely  on  the  white  than  on  the 
brown  paper. 

Mrs,  B.  The  lens  collects  an  equal  number  of  rays 
to  a  focus,  whether  you  hold  the  white  or  the  brown  pa- 
per there  ;  but  the  white  paper  appears  more  luminous 
in  the  focus,  because  most  of  the  rays,  instead  of  en- 
tering into  the  paper,  are  reflected  by  it;  and  this  is  the 
reason  that  the  paper  is  not  burnt :  whilst  on  the  con- 
trary, the  brown  paper,  which  absorbs  more  light  than 
it  reflects,  soon  becomes  heated  and  takes  fire. 

Caroline.  This  is  extremely  curious  ;  but  why  should 
brown  paper  absorb  more  rays  than  white  paper  ? 

Mrs.  B.  lam  far  from  being  able  to  give  a  satisfac- 
tory answer  to  that  question.  We  can  form  but  mere 
conjecture  on  this  point ;  and  suppose  that  the  tenden- 
cy to  absorb,  or  reflect  rays,  depends  on  the  arrange- 
ment of  the  minute  particles  of  the  body,  and  that  this 
diversity  of  arrangement  renders  some  bodies  suscepti- 
ble of  reflecting  one  coloured  ray,  and  absorbing  the 
others  ;  whilst  other  bodies  have  a  tendency  to  reflect 
all  the  colours,  and  others  again,  to  absorb  them  all. 


284  ON  REFRACTION  AN13  COLOURS. 

Emily,  And  how  do  you  know  which  colours  bodies 
have  a  tendency  to  reflect ;  or  which  to  absorb  ? 

Mrs,  B,  Because  a  body  always  appears  to  be  of  the 
colour  which  it  reflects  ;  for,  as  we  see  only  by  reflected 
rays,  it  can  appear  but  of  the  colour  of  those  rays. 

Caroline.  But  we  see  all  bodies  of  their  our  natural 
colour,  Mrs.  B. ;  the  grass  and  trees,  green  ;  the  sky, 
blue ;  the  flowers,  of  various  hues. 

Mrs,  B,  True  ;  but  why  is  the  grass  green  ?  be- 
cause it  absorbs  all  except  the  green  rays  ;  it  is  there- 
fore  these  only  which  the  grass  and  trees  reflect  to  our 
eyes,  and  which  makes  them  appear  green.  The  sky 
and  flowers  in  the  same  manner,  reflect  the  various 
colours  of  which  they  appear  to  us  ;  the  rose,  the  red 
rays;  the  violet,  the  blue;  the  jonquil,  the  yellow,  &c. 

Caroline,  But  these  are  the  permanent  colours  of 
the  grass  and  flowers,  whether  the  sun's  rays  shine  on 
them  or  not. 

Mrs,  B,  Whenever  you  see  those  colours,  the  flow- 
ers must  be  illumined  by  some  light ;  and  light,  from 
whatever  source  it  proceeds,  is  of  the  same  nature, 
composed  of  the  various  coloured  rays,  which  paint  the 
grass,  the  flowers,  and  every  coloured  object  in  nature. 

Caroline,  But,  Mrs.  B.,  the  grass  is  green,  and  the 
flowers  are  coloured,  whether  in  the  dark,  or  exposed  to 
the  light? 

Mrs,  B,     Why  should  you  think  so  ? 

Caroline,     It  cannot  be  otherwise. 

Mrs,  B,  A  most  philosophical  reason  indeed  !  But, 
as  T  never  saw  them  in  the  dark,  you  will  allow  me  to 
dissent  from  your  opinion. 

Caroline,  What  colour  do  you  suppose  them  to  be, 
then,  in  the  dark  ? 


ON  ItEFHACTION  AND  COLOURS.  285 

J^frs,  B,    None  at  all ;  or  bl^ck,  which  is  the  same 

ing.     You  can  never  see  objects  without  light.    Light 

composed  of  colours,  therefore  there  can  be  no  light 
without  colours ;  and  though  every  object  is  black,  or 
without  colour  in  the  dark,  it  becomes  coloured,  as  soon 
as  it  becomes  visible.  It  is  visible,  indeed,  but  by  the 
coloured  rays  which  it  reflects  )  therefore  we  can  see  it 
only  when  coloured. 

Caroline.  All  you  say  seems  very  true,  and  I  know 
not  what  to  object  to  it ;  yet  it  appears  at  the  same  time 
incredible  !  What,  Mrs.  B.,  are  we  all  as  black  as  ne- 
groes, in  the  dark  .^  you  make  me  shudder  at  the  thought. 

Mrs,  B,  Your  vanity  need  not  be  alarmed  at  the 
idea,  as  you  are  certain  of  never  being  seen  in  that 
state. 

Caroline,  That  is  some  consolation,  undoubtedly ; 
but  what  a  melancholy  reflection  it  is,  that  all  nature 
which  appears  so  beautifully  diversified  with  colours 
should  be  one  uniform  mass  of  blackness! 

Mrs,  B,  Is  nature  less  pleasing  for  being  coloured, 
as  well  as  illumined  by  the  rays  of  light;  and  are 
colours  less  beautiful,  for  being  accidental,  rather  than 
essential  properties  of  bodies  } 

Providence  appears  to  have  decorated  nature  with 
the  enchanting  diversity  of  colours,  which  we  so  much 
admire,  for  the  sole  purpose  of  beautifying  the  scene, 
and  rendering  it  a  source  of  pleasurable  enjoyment:  it 
is  an  ornament  which  embellishes  nature,  whenever  we 
behold  her.  What  reason  is  there  to  regret  that  shie 
does  not  wear  it  when  she  is  invisible  ? 

Emily,  I  confess,  Mrs,  B.,  that  I  have  had  ray 
doubts,  as  well  as  Caroline,  though  she  has  spared  me 
the  pains  of  expressing  them;  but  I  have  just  thought 


286      ON  REFRACTION  AND  COLOURS. 

of  an  expeiitnent.  which, if  it  succecdsj,  will,  I  am  sure 
satisfy  us  both.  It  is  certain,  that  we  cannot  see  bodies 
in  the  dark,  to  know  whether  they  have  then  any  colour. 
But  we  may  place  a  coloured  body  in  a  ray  of  li^ht, 
which  has  been  refracted  by  a  prism  ;  and  if  your  the* 
ory  is  true,  the  body,  of  whatever  colour  it  naturally  is 
must  appear  of  the  colour  of  the  ray  in  which  it  is 
placed  ;  for  since  it  receives  no  other  coloured  rays,  it 
can  reflect  no  others, 

Caroline,  Oh  !  that  is  an  excellent  thought,  Emily; 
will  you  stand  the  test  Mrs.  B.  ? 

Mrs,  IL  I  consent :  but  we  must  darken  the  room, 
and  admit  only  the  ray  which  is  to  be  refracted  ;  other- 
wise, the  white  rays  will  be  reflected  on  the  body  under 
trial,  from  various  parts  of  the  room.  With  what  do 
you  choose  to  make  the  experiment  ? 

Caroline.  This  rose :  look  at  it,  Mrs.  B.,  and  tell 
me  whether  it  is  possible  to  deprive  it  of  its  beautiful 
colour  ? 

Mrs.  B,  We  shall  see.' — I  expose  it  first  to  the  red 
rays,  and  the  flower  appears  of  a  more  brilliant  hue ; 
but  observe  the  green  leaves— 

Caroline,  They  appear  neither  red  nor  green  ;  but 
of  a  dingy  brown  with  a  reddish  glow  ! 

Mrs.  B.  They  cannot  be  green,  because  they  have 
no  green  rays  to  reflect ;  neither  are  they  red,  because 
green  bodies  absorb  most  of  the  red  rays.  But  though 
bodies,  from  the  arrangement  of  their  particles,  have  a 
tendency  to  absorb  some  rays,  and  reflect  others,  yet  it 
is  not  natural  to  suppose,  that  bodies  are  so  perfectly 
uniform  in  their  arrangement,  as  to  reflect  only  pure 
rays  of  one  colour,  and  perfectly  absorb  the  others  ;  it 
\found,  on  the  contrary,  that  a  body  reflects,  in  great 


ON  REFRACTION  AND  COLOURS.      287 

abundance,  the  rays  which  determine  its  colour,  and  the 
others  in  a  greater  or  less  degree,  in  proportion  as  they 
are  nearer  or  further  from  its  own  colour,  in  the  order 
of  refrangihility.  The  green  leaves  of  the  rose,  there- 
fore, will  reflect  a  few  of  the  red  rays,  which,  blended 
with  their  natural  blackness,  give  them  that  brown 
tinge :  if  they  reflected  none  of  the  red  rays,  they 
would  a[)pear  perfectly  black.  Now  I  shall  hold  the  rose 
in  the  blue  rays — 

Caroline.  Oh,  Emily,  Mrs.  B.  is  right !  look  at  the 
rose  :  it  is  no  longer  red,  but  of  a  dingy  blue  colour. 

Emily.  This  is  the  most  wonderful  of  any  thing  we 
have  yei  learnt.  But,  Mrs.  B.,  what  is  the  reason  that 
the  green  leaves  are  of  a  brighter  blue  than  the  rose  ? 

Mrs.  B.  The  green  leaves  reflect  both  blue  and 
yellow  rays,  which  produces  a  green  colour.  They  are 
now  in  a  coloured  ray,  which  they  have  a  tendency  to 
reflect;  thoy,  therefore,  reflect  more  of  the  blue  rays 
than  the  rose,  (which  naturally  absorbs  that  colour,) 
and  will,  of  course,  appear  of  a  brighter  blue. 

Emilij.  Yet,  in  passing  the  rose  through  the  dif- 
ferent colours  of  the  spectrum,  the  flower  takes  them 
more  readily  than  the  leaves. 

Mrs.  B.  Because  the  flower  is  of  a  paler  hue.  Bodies 
which  reflect  all  the  rays  are  white ;  those  which  ab- 
sorb them  all  are  black  :  between  these  extremes,  the 
body  appears  lighter  or  darker,  in  proportion  to  the 
quantity  of  rays  they  reflect  or  absorb.  This  rose  is  of 
a  pale  red  :  it  approaches  nearer  to  white  than  black ; 
it  therefore  reflects  rays  more  abundantly  that  it  ab- 
sorbs them. 

Emily.    But  if  a  rose  has  so  strong  a  tendency  to 


288  ON  REFRACTION  AND  COLOURS. 

reflect  rajs,  I  should  have  imagined  that  it  would  be  of 
a  deep  red  colour. 

Mrs,  B,  I  mean  to  say,  that  it  has  a  general  ten- 
dency to  reflect  rays.  Pale-coloured  bodies  reflect  all 
the  coloured  rays  to  a  certain  degree,  which  produces 
their  paleness,  approaching  to  whiteness :  but  one 
colour  they  reflect  more  than  the  rest;  this  predomi- 
nates over  the  white,  and  determines  the  colour  of  the 
body.  Since,  then,  bodies  of  a  pale  colour  in  some  de- 
gree reflect  all  the  rays  of  light,  in  passing  through  the 
various  colours  of  the  spectrum,  they  will  reflect  them 
all  with  tolerable  brilliancy ;  but  will  appear  most  vivid 
in  the  ray  of  their  natural  colour.  The  green  leaves,  on 
the  contrary,  are  of  a  dark  colour,  bearing  a  stronger 
resemblance  to  black,  than  to  white  ;  they  have,  there- 
fore, a  greater  tendency  to  absorb,  than  to  reflect  rays; 
and  reflecting  very  few  of  any  but  the  blue  and  yellow 
rays,  they  will  appear  dingy  in  passing  through  the 
other  colours  of  the  spectrum. 

Caroline,  They  must,  however,  reflect  great  quan- 
tities of  the  green  rays,  to  produce  so  deep  a  colour. 

Mrs,  B,  Deepness  or  darkness  of  colour  proceeds 
rather  from  a  deficiency  than  an  abundance  of  reflect- 
ed rayts.  Remember  that  bodies  are,  of  themselves, 
black  ;  and  if  a  body  reflects  only  a  few  green  rays,  it 
will  appear  of  a  dark  green;  it  is  the  brightness  and 
intensity  of  the  ^'olour  which  show  that  a  great  quan 
tity  of  rays  are  reflected. 

Emily,  A  white  body,  then,  which  reflects  all  the 
rays,  will  appear  equallv  bright  in  all  the  colours  of  the 
spectrum. 

Mrs.   B.     Certainly.     And  this  is  easily  proved  by 


ON  REFRACTION  AND  COLOURS.  289 

passing  a  sheet  af  white  paper  through  the  rays  of  the 
spectrum. 

Caroline,  What  is  the  reason  that  blue  often  ap- 
pears green  by  cantlle-lighi  ? 

Mrs.  B,  The  light  of  a  candle  is  not  so  pure  as 
that  of  the  sun  :  it  has  a  yellowish  tinge,  and  when  re- 
fracted by  the  prism,  the  yellow  rays  predominate  ;  and 
as  blue  bodies  reflect  the  yellow  rays  in  the  next  pro- 
portion, (!)eing  next  in  order  of  refrangibility),  the 
superabundance  of  yellow  rays  gives  to  blue  bodies  a 
greenish  hue. 

Caroline.  Candle-light  must  then  give  to  all  bodies 
a  yellowish  tinge,  from  the  excess  of  yellow  rays;  and 
yet  it  is  a  common  remark,  that  people  of  a  sallow 
complexion  appear  fairer  or  whiter  by  candle-light. 

Mrs.  B,  The  yellow  cast  of  their  complexion  is  not 
so  striking,  when  evevy  object  has  a  yellow  tinge. 

Emily,  Prays  why  does  the  sun  appear  red  through 
a  fog  ? 

Mrs.  B,  It  is  supposed  to  be  owing  to  the  red  rays 
having  a  greater  momentum,  which  gives  them  power 
to  traverse  so  dense  an  atmosphere.  For  the  same  rea- 
son, the  sun  generally  appears  red  at  rising  and  sitting  ; 
as  the  increased  quantity  of  atmosphere,  which  the 
oblique  rays  must  traverse,  loaded  with  the  mists  and 
vapoursj  which  are  usually  formed  at  those  times  pre- 
vents the  other  rays  from  reaching  us. 

Caroline.  And,  pray,  why  are  the  skies  of  a  blue 
colour  ? 

Mrs.  B.     You  should  rather  say,  the  atmosphere ;  for 
the  sky  is  a  very  vague  term,  thf  m  aning  of  which  it 
would  be  difficult  to  define  philosopiucally. 
Bb 


-90  ON  REFRACTION  AND  COLOURS. 

Caroline.  But  the  colour  of  the  atmosphere  should 
be  white,  since  all  the  rays  traverse  it  in  their  passage 
to  the  earth. 

Mrs,  B,  Do  not  forget  that  we  see  none  of  the  rays 
which  pass  from  the  sun  to  the  earth,  excepting  those 
which  meet  our  eyes ;  and  this  happens  only  if  we 
look  at  the  sun,  and  thus  intercept  the  rays,  in  which 
case,  you  know,  the  sun  appears  white.  The  atmos- 
phere is  a  transparent  medium,  through  which  tiie  sun's 
rays  pass  freely  to  t\\e.  earth  ;  but  when  reflected  back 
into  the  atmosphere,  their  momentum  is  considerably 
diminished ',  and  they  have  not  all  of  them  power  to 
travei'se  it  a  second  time.  The  momentum  of  the 
blue  rays  is  least ;  these,  therefore,  are  the  most  impeded 
in  their  return,  and  are  chleliy  reflected  by  the  atmos- 
phere;  this  reflection  is  performed  in  ^\qv'^  possible  di- 
rection ;  so  that  whenever  we  look  at  the  atriu)s»>l;ere, 
some  of  tnese  rays  fall  upon  our  eyes  ;  hence  we  see 
the  air  of  a  blue  colour.  U  tije  atmosphere  did  not  re- 
ilect  any  rays,  thoujjji  i:i\Q,  objects  on  the  surface  of  tlie 
earth  v/ould  be  illumined,  t\\{i  skies  would  appear  per- 
fectly black. 

Caroline,  OJ!,  liow  melancljoiy  that  would  be ;  and 
how  pernicious  to  the  sit;,lit,  to  be  constantly  viewing 
bri::':ht  obje'ts  against  a  black  sky.  But  what  is  the  rea- 
son tliat  bodies  offen  change  tlieir  colour;  as  leaves 
which  wither  in  autumn,  or  a  spot  of  itjk  which  pro- 
duces an  iron-mouid  on  linen  .^ 

Mrs,  B,  It  arises  from  some  chemical  change,  \yhich 
takes  place  in  the  internal  arrangement  of  the  parts,  by 
which  they  lose  their  tender.cy  to  reflect  certain  colours, 
and  acquire  the  power  of  reflecting  others.  A  withered 
leaf  tiius  no  longer  reliects  the  blue  rays;  it  appears^ 


ON  REFRACTION  AND  COLOURS.  291 

therefore,  yellow,  or  has  a  slight  tendency  to  reflect 
several  rays  which  produce  a  dingy  brown  colour. 

An  ink-spot  on  linen  at  first  absorbs  all  the  rays ; 
but,  exposed  to  the  air,  it  undergoes  a  chemical  change, 
and  the  spot  partially  regains  its  tendency  to  reflect 
colours,  but  with  a  preference  to  reflect  the  yellow  rays, 
and  such  is  the  colour  of  the  iron-mould. 

Emily,  Bodies,  then,  far  from  being  of  the  colour 
which  they  appear  to  possess,  are  of  that  colour  which 
they  have  the  greatest  aversion  to,  which  they  will  not 
incorporate  with,  but  reject  and  drive  from  them. 

Mrs.  B,  It  certainly  is  so  ;  though  I  scarcely  dare 
venture  to  advance  such  an  opinion,  whilst  Caroline  is 
contemplating  her  beautiful  rose. 

Caroline,  My  poor  rose  !  you  are  not  satisfied 
with  depriving  it  of  colour,  but  even  make  it  have  an 
aversion  to  it;  and  I  am  unable  to  contradict  you. 

Emily,  Since  dark  bodies  absorb  more  solar  rays 
than  light  ones,  the  former  should  sooner  be  heated  if 
exposed  to  the  sun  ? 

Mrs,  B,  And  they  are  found  by  experience  to  be  so. 
Have  you  never  observed  a  black  dress  to  be  warmer 
than  a  white  one  ? 

Emily,  Yes,  and  a  white  one  more  dazzling:  the 
black  is  heated  by  absorbing  the  rays,  the  white  daz- 
zling by  reflecting  them. 

Caroline,  And  this  was  the  reason  that  the  brown 
paper  was  burnt  in  the  focus  of  the  lens^  whilst  the 
white  paper  exhibited  the  most  luminous  spot,  but  did 
not  take  fire. 

Mrs,  B,  It  was  so.  It  is  now  full  time  to  conclude 
our  lesson.  At  our  next  meeting,  I  shall  givi  you  a 
description  of  the  eye. 


CONVERSATION  X  Vll. 


OPTICS. 

©N    THE    STRUCTURE    OF   THE    EYE,   AND    OPTIGAL 
INSTRUMENTS. 

DESCRIPTION   or   THE    EXE. OF    THE    IMAGE    OK   THE     HETINA- — 

fiEPRACTION    OF    THE    HUMOURS    OF    THE     EYE, OF     THE   USE   OS 

SPECTACLES. OF    THE    SI]VGLE    MICROSCOPE. OF   THE     BOUBLE 

MICROSCOPE. OF    THE    SOLAR   MICROSCOPE.— MAGIC   LASTH0RI7. 

BEFRACTIKG   TELESCOPE. REFLECTING   TELESCOPE. 


Mrs.  B. 

The  body  of  the  eye  is  of  a  spherical  form:  (fig.  1. 
plate  XXI,)  it  has  two  membranous  coverings;  the  ex- 
ternal one,  a  a  a,h  called  the  sclerotica  :  this  has  a  pro- 
jection in  that  part  of  the  eye  which  is  exposed  to  view, 
b  h,  which  is  called  the  cornea,  because,  when  dried,  it 
has  nearly  the  consistence  of  very  fine  horn,  and  is 
sufficiently  transparent  for  the  light  to  obtain  free  pas- 
sage through  it. 

The  second  membrane  which  lines  the  cornea,  and 
envelopes  the  eye,  is  called  the  choroid,  cec  ;  this  has 


Pt.^te:xxj. 


Tii1>.  Iry  J.THinujfht'iysTlalad? 


OPTICS  295 

an  opening  in  front,  just  beneath  the  cornea,  which 
forms  the  pupil,  d  d,  throuo;h  which  the  rajs  of  light  pass 
into  the  eye.  The  pupil  is  surrounded  by  a  coloured 
border,  called  the  iris,  e  e,  which,  by  its  muscular  mo- 
tion, always  preserves  the  pupil  of  a  circular  form, 
whether  it  is  expanded  in  the  dark,  or  contracted  by  a 
strong  lii^ht.  This  you  will  understand  better  by 
examining  fis;.  2. 

Emilif,  I  did  not  know  that  the  pupil  was  suscepti- 
ble of  varying  its  dimensions. 

Mrs,  B,  The  construction  of  the  eye  is  so  admirable^ 
that  it  is  capable  of  adapting  itself,  more  or  less,* to  th^ 
circumstances  in  wliich  it  is  placed.  In  a  faint  light  the 
pupil  dilates  so  as  to  receive  an  additional  quantity  of 
rays,  and  in  a  strong  light  it  contracts,  in  order  to  pre- 
vent the  intensity  ( f  the  light  from  injuring  the  optic 
nerve.  Observe  Emily's  eyes,  as  she  sits  looking  to- 
wards the  windows:  her  pupils  appear  very  small,  and 
the  iris  large.  Now,  Emily,  turn  from  the  light,  and 
cover  your  eyes  with  your  hand,  so  as  entirely  to  ex- 
elude  it  for  a  few  moments. 

Caroline,  How  very  much  the  pupils  of  her  eyes  are 
now  enlarged,  and  the  iris  diminished.  This  is,  no 
doubt  the  reason  why  the  eyes  suff'er  pain,  when  from 
darkness  they  suddenly  come  into  a  strong  light ;  for 
i\\Q  pupil  being  dilated,  a  quantity  of  rays  must  rush 
in  before  it  has  time  to  contract. 

Emily,  And  when  we  go  from  a  strong  light  into 
obscurity,  we  at  first  imagine  ourselves  in  total  dark- 
ness ;  for  a  sullicient  number  of  rays  cnnnot  gain  ad- 
mitance  into  the  contracted  pupil,  to  enable  us  to 
distinguish  objects :  but  in  a  few  minutes  it  dilates, 
B  b  2 


294  OPTICS. 

and  we  clearly  perceive  objects  which  were  before  in- 
visible. 

Mrs.  B.  It  is  just  so.  The  choroid  c  c,is  imbued  with 
a  black  liquor,  which,  serves  to  absorb  all  the  rays  that 
are  irregularly  reflected,  and  to  convert  the  body  of 
the  eye  into  a  more  perfect  camera  obsura.  When  the 
pupil  is  expanded  to  its  utmost  extent,  it  is  capable  of 
admitting  ten  times  the  quantity  of  light  that  it  does- 
when  most  contracted.  In  cats,  and  animals  which  are 
said  to  see  in  the  dark,  the  power  of  dilatation  and 
contraction  of  the  pupil  is  still  greater  :  it  is  computed  . 
that  their  pupils  may  receive  one  hundred  times  more 
light  at  one  time  than  at  another. 

Within  these  coverings  of  the  eye-ball  are  contained 
three  transparent  substances,  called  humours.  The  first 
occupies  the  space  immediately  behind  the  cornea,  and 
is  called  the  aqueous  humour,  //,  from  its  liquidity 
and  its  resemblance  to  water.  Beyond  this  is  situated 
the  crystalline  humour, g*  ^,  so  called  from  its  clearness 
and  transparency  :  it  has  the  form,  of  a  lens,  and  re- 
fracts the  rays  of  ligV»t  in  a  greater  degree  of  perfection 
than  any  that  have  been  constructed  by  art:  itisattach- 
v^d  by  two  muscles,  ?/i  m,  to  each  side  of  the  choroid. 
The  back  part  of  the  eye,  between  the  crystalline  hu- 
mour and  the  retina,  is  filled  by  the  vitreous  humour, 
h  h,  which  derives  its  name  from  a  resemblance  it  is 
supposed  to  bear  to  glass  or  vitrified  substances. 

The  membranous  coverings  of  the  eye  are  intended 
chiefly  for  the  preservation  of  the  retina,  ii,  which  is 
by  far  the  most  important  part  of  the  eye,  as  it  is  that 
which  receives  the  impression  of  the  objects  of  sight, 
and  conveys  it  to  the  mind.    The  retina  consists  of  an 


0PTICS.  295 

expansion  of  the  optic  nerve,  of  a  most  perfect  white- 
ness :  it  proceeds  from  the  brain,  enters  the  eje,  at  n,  on 
the  side  next  the  nose,  and  is  finely  spread  over  the  in- 
terior surface  of  the  choroid. 

The  j-ajs  of  light  which  enter  the  eje  by  the  pupil 
are  refracted  by  the  several  humours  in  their  passage 
through  them,  and  unite  in  a  focus  on  the  retina. 

Caroline,  I  do  not  understand  the  use  of  these  re- 
fracting humours  :  the  image  of  objects  is  represented  in 
the  camera  obscura,  without  any  such  assistance. 

Jlrs.  i?.  That  is  true ;  but  the  representation  would 
be  much  more  strong  and  distinct,  if  we  enlaro-e  the 
opening  of  the  camera  obscura,  and  receive  the  rays 
into  it  through  a  lens. 

1  have  told  you  that  rays  proceed  from  bodies  in  all 
possible  directions.  We  must,  therefore,  consider  every 
part  of  an  object  which  sends  rays  to  our  eyes,  as  points 
from  which  the  rays  diverge,  as  from  a  centre. 

Emily.  These  divergent  rays,  issuing  from  a  single 
point,  1  believe  you  told  us,  were  called  a  pencil  of 
rays? 

Mrs.  B.  Yes.  Now,  divergent  rays,  on  entering  the 
pupil,  do  not  cross  each  other;  the  pupil,  however,  is 
sufficiently  large  to  admit  a  small  pencil  of  them  ;  and 
these,  if  not  refracted  to  a  focus  by  the  humours,  would 
continue  diverging  after  they  had  passed,  the  pupil, 
would  fall  dispersed  upon  the  retina,  and  thus  the  image 
of  a  single  point  would  be  expanded  over  a  large  por- 
tion of  the  retina.  The  divergent  rays  from  every  other 
point  of  the  object  would  bespread  over  a  similar  extent 
of  space  and  would  interfere  and  be  confounded  with 
the  first ;  so  that  no  distinct  image  could  be  formed,  and 


296  OPTICS. 

the  retina  would  represent  total  confusion  both  of  figure 
and  colour.  Fig  3.  represents  two  pencils  of  rajs  issuing 
from  two  points  of  the  tree  A  B,  and  entering  the  pu- 
pil C,  refracted  by  the  crystalline  humour  D,  and  form- 
ing distinct  images  of  the  spot  they  proceed  from,  on  the 
retina  at  a  h.  Fig.  4.  differs  from  the  preceding,  merely 
from  not  being  supplied  with  a  lens  ;  in  consequence  of 
which  the  pencils  of  rays  are  not  refracted  to  a  focus, 
and  no  distinct  image  is  formed  on  the  retina.  1  have 
delineated  only  the  rays  issuing  from  two  points  of  an 
object,  and  distinguished  the  two  pencils  in  fig.  4.  by 
describing  one  of  them  with  dotted  lines  :  the  inter- 
ference of  these  two  pencils  of  rays  on  the  retina  will 
enable  you  to  form  an  idea  of  the  confusion  which 
would  arise,  from  thousands  and  millions  of  points  at 
the  same  instant  pouring  their  divergent  rays  upon  the 
retina. 

Emily.  True  ;  but  I  do  not  yet  well  understand  how 
the  refracting  humours  remedy  this  imperfection. 

Mrs,  B.  The  refraction  of  these  several  humours 
unite  the  whole  of  a  pencil  of  rays,  proceeding  from  any 
one  point  of  an  object,  to  a  corresponding  point  on  the 
retina,  and  the  image  is  thus  rendered  distinct  and 
strong.  If  you  conceive,  in  fig.  5.,  every  point  of  the 
ivQQ  to  send  forth  a  pencil  of  rays  similar  to  those,  A  B, 
every  part  of  the  tree  will  be  as  accurately  represented 
on  the  retina  as  the  points  a  b, 

Emily,  How  admirably,  how  wonderfully,  this  is 
contrived  ! 

Caroline,  But  since  the  eye  requires  refracting  hu- 
mours in  order  to  have  a  distinct  representation  formed 


OPTICS.  297 

©n  the  retina,  why  Is  not  the  same  refraction  necessary 
for  the  iina^e  formed  in  the  camera  ob*cura  ? 

Mffi,  B.  Because  the  aperture  through  which  we 
received  the  rays  into  the  camera  obscura  is  extremely 
small ;  st)  that  but  very  few  of  the  rays  diverging  from  a 
point  gain  admittance;  but  we  vviil  now  enlarge  the 
aperture,  and  furnish  it  with  a  lens,  and  you  will  find 
the  landscape  to  be  more  perfectly  represented. 

Caroline.  How  obscure  and  confused  the  image  is 
now  that  you  have  enlarged  the  opening,  without  put- 
ting in  the  lens. 

Mrs.  B.  Such,  or  very  similar,  would  be  the  re- 
presentation on  the  retina,  unassisted  by  the  refracting 
humours.  But  see  what  a  difference  is  produced  by  the 
introduction  of  the  lens,  which  collects  each  pencil  of 

divergent  rays  into  their  several  foci. 

Caroline,  The  alteration  is  wonderful :  the  represen- 
tation is  more  clear,  vivid,  and  beautiful  than  ever. 

Mrs.  B.  You  will  now  be  able  to  understand  the 
nature  of  that  imperfection  of  sight,  which  arises  from 
the  eyes  being  too  prominent.  In  such  cases,  the  crys- 
talline humour,  D,  (fig.  5.)  being  extremely  convex,  re- 
fracts the  rays  too  much,  and  collects  af  pencil,  pro- 
ceeding from  the  object  A  B,  into  a  focus,  F,  before 
they  reach  the  retina.  From  this  focus  the  rays  pro- 
ceed diverging,  and  consequently  form  a  very  confused 
image  on  the  retina  at  a  b.  This  is  the  defect  of  short- 
sighted people. 

Emily,  I  understand  it  perfectly.  But  why  is  this 
defect  remedied  by  bringing  the  object  nearer  to  the  eye, 
as  we  find  to  be  tlie  case  with  shgrt-sighted  people? 

Mrs,  B,     The  nearer  you  bring  an  object  to  your  eye 


298  OPTICS. 

the  more  divergent  the  rajs  fall  upon  the  crystalline 
humour,  and  they  are  consequently  not  so  soon  con- 
verged to  a  focus  :  this  focus  therefore,  either  falls  up- 
on the  retina,  or  at  least  approaches  nearer  to  it,  and 
the  object  is  proportionably  distinct,  as  in  fig.  6. 

Emily,  The  nearer,  then,  you  bring  an  object  to  a 
lens  the  further  the  image  recedes  behind  it. 

Mrs»  B,  Certainly.  But  short-sighted  persons  have 
another  resource  for  objects  which  they  cannot  approach 
to  their  eyes  ;  this  is  to  place  a  concave  lens,  C  D,  (fig. 
1.  plate  XXII.)  before  the  eye,  in  order  to  increase  the 
divergence  of  the  rays.  The  effect  of  a  concave 
lens  is  you  know  exactly  the  reverse  of  a  convex 
one :  it  renders  parallel  rays  divergent,  and  those 
which  are  already  divergent,  still  more  so.  By  the  as- 
sistance of  such  glasses  therefore,  the  rays  from  a  dis- 
tant object  fall  on  the  pupil,  as  divergent  as  those  from 
a  less  distant  object ;  and,  with  short-sighted  people, 
they  throw  the  image  of  a  distant  object  back  as  far  as 
the  retina. 

Caroline,     This  is  an  excellent  contrivance,  indeed. 

Mrs,  B,  And  tell  me,  what  remedy  would  you  de- 
vise for  such  persons  as  have  a  contrary  defect  in  their 
sight;  that  is  to  say,  in  whom  the  crystalline  humour, 
being  too  flat,  does  not  refract  the  rays  sufficiently,  so 
that  they  reach  the  retina  before  they  are  converged 
to  a  point? 

Cariidne.  I  suppose  that  a  contrary  remedy  must 
be  applied  to  this  ^^^^^'zci ;  that  is  to  say,  a  convex  lens, 
L  M,  %.  2.,  to  make  up  for  the  deficiency  of  convexity 
of  tlie  crystalline  humour,  OP.  For  the  conve>:  lens 
would  bring  the  rays  nearer  together,  so  that  they 
would  fall  either  less  divergent,  or  parallel  on  the  crys- 


PiATE^Xn. 


Ihih.  7>j  J.T.SimrphiYVi^ FJnlad? 


OPTICS.  i99 

talline  humour;  and,  by  being  sooner  converged   to  a 
focus,  would  fall  on  the  retina. 

Jh^s,  B.  Very  well,  Caroline.  Tliis  is  the  reason 
why  elderly  people,  the  humours  of  whose  eyes  are  de- 
cayed by  age,  are  under  tlie  necessity  of  using  convex 
spectacles.  And  when  deprived  of  that  resource,  they 
hold  the  object  at  a  distance  from  their  eyes,  as  in  ng. 
4,  in  order  to  bring  the  focus  forwarder. 

Caroline.  I  have  often  been  surprized,  when  my 
grandfather  reads  without  his  spectacles,  to  see  him  hold 
the  book  at  a  considerable  distance  from  his  eyes.  But 
I  now  understand  it ;  for  the  more  distant  the  object 
is  from  the  crystalline,  the  nearer  (he  image  will  be  to  it. 

Emily,  I  comprehend  the  nature  of  these  two  opposite 
defects  very  well ;  but  I  cannot  now  conceive,  how  any 
sight  can  be  perfect:  for  if  the  crystalline  humour  is  of 
a  proper  degree  of  convexity,  to  bring  the  image  of 
distant  objects  to  a  focus  on  the  retina,  it  will  not  re- 
present near  objects  distinctly  ;  and  if,  on  the  contrary, 
it  is  adapted  to  give  a  clear  image  of  near  objects,  it  will 
produce  a  very  imperfect  one  of  distant  (objects. 

Mrs.  B.  Your  obsorvation  is  vevy  good,  Emily  ;  and 
it  is  true,  tliat  every  person  would  be  subject  to  one  of 
these  two  defects,  if  we  had  it  not  in  our  power  to  in- 
crease or  diminish  the  convexity  of  the  crystalline  hu- 
mour, and  to  project  it  towards,  or  draw  it  back  from 
the  objt^ot,  as  circuiDstances  require.  In  a  young  well- 
■  constructed  eye,  the  two  muscles  to  which  the  cry-stal- 
line  humour  is  attav;hed  have  so  perfect  a  command 
over  it,  that  the  focus  of  the  rays  constantly  falls  on  the 
retina,  and  an  image  is  formed  ecjually  distinct  both  of 
distant  objects  and  of  those  which  are  near. 


300  OPTiCS. 

Caroline.  In  the  eyes  oi  fishes,  which  are  the  only 
eyes  I  have  ever  seen  separate  fioiii  the  head,  the 
cornea  does  not  pfotrude,  in  that  part  of  the  eye  which 
is  exposed  to  view. 

Mrs,  B.  The  cornea  of  the  eye  of  a  fish  is  not  more 
convex  than  the  rest  of  the  ball  of  the  eye  ;  but  to  sup- 
ply this  deficiency,  tlieir  crystalline  humour  i:>  sjiheri- 
cal,and  refracts  the  rays  so  much,  that  it  does  not  re- 
quire the  assistance  of  the  cornea  to  bri«g  them  to  a 
focus  on  the  retina. 

Emili},  Pray,what  is  the  reason  that  we  cannot  see 
an  object  distinctly,  if  we  approach  it  sery  near  to  the 
eye  ? 

Mrs.  B.  Because  the  rays  fall  on  the  crystalline 
humour  too  divergent  to  be  refracted  to  a  focus  on  the 
retina,  the  confusion,  therefore,  arising  from  viewing  an 
object  too  near  the  eye,  is  similar  to  that  which  pro- 
ceeds fiom  a  flattened  crystalline  humour ;  the  rays 
reach  the  retina  before  they  are  collected  to  a  focus, 
(fig.  4.)  If  it  were  not  for  this  imperfection,  we  ^^hould 
be  able  to  see  and  distinguish  the  parts  of  objects, 
which  are  now  invisible  to  us  from  their  minuteness; 
for  could  we  approach  them  very  near  the  ey?^,  their 
image  ori  the  retina  would  be  so  much  magnified  as  to 
render  them  visible. 

Emily,  And  could  there  be  no  contrivance  to  con- 
vey the  rays  of  objects  viewed  close  to  the  eye,  so  that 
they  should  be  refracted  to  a  focus  on  the  retina  ? 

Mrs,  B,  The  microsco})e  is  constructed  for  this 
purpose.  The  single  microscope  (iig.  5.)  consists  sim- 
ply of  a  convex  lens,  coiitmonly  called  a  magnifying 
glass ;  in  the  focus  of  which  the  object  is  placed,  and 


OPTICS.  3©1 

through  which  it  is  viewed  :  bj  this  means,  you  are  en- 
abled to  approach  your  eye  very  near  the  object,  for  the 
lens  A  B,  by  diminishing  the  divergence  of  the  rays,  be- 
fore they  enter  the  pupil  C,  makes  them  fall  parallel 
on  the  crystalline  humour  D,  by  which  they  are  refract- 
ed to  a  focus  on  the  retina,  at  R  R. 

Emily.  This  is  a  most  admirable  invention,  and  no- 
thing can  be  more  simple,  for  the  lens  magnifies  the  ob- 
ject merely  by  allowing  us  to  bring  it  nearer  to  the  eye. 

Mrs.  B,  Those  lenses,  therefore,  which  have  the 
shortest  focus  will  ifiagnify  the  object  most,  because 
they  enable  us  to  bring  the  object  nearest  to  the  eye. 

Emibj.  But  a  lens,  that  has  the  shortest  focus,  is 
most  bulging  or  convex  ;  and  the  protuberance  of  the 
lens  will  prevent  the  eye  from  approaching  very  near 
to  the  object. 

Mrs.  B.  This  is  remedied  by  making  the  lens  ex- 
tremely small  :  it  may  then  be  spherical  without  occu- 
pying much  space,  and  thu^  unite  the  advantages  of  a 
short  focus,  and  of  allowing  the  eye  to  approach  the 
object. 

Caroline.  We  have  a  microscope  at  home,  which  is 
a  much  more  complicated  instrument  than  that  you 
have  described. 

Mrs.  B.  It  is  a  double  microscope  (fig.  6.),  in  which 
you  see,  not  the  object  A  B,  but  a  magnified  image  of 
it,  ft /;.  In  this  microscope,  two  lenses  are  employed, 
the  one,  L  M,  for  the  pifrpose  of  magnifying  the  object, 
i§  called  the  object  glass;  the  other,  N  O,  acts  on  the 
principle  of  the  single  microscope,  and  is  called  the 
eye-«rlass. 

There  is  another  kind  of  microscope,  called  the  solar 
c  c 


302  OPTICS. 

microscope,  which  is  the  most  wonderful  from  its  great 
magnifying  power :  in  this  we  also  view,  an  image  formed 
by  a  lens,  not  the  object  itself.  As  the  sun  shines,  I 
can  show  you  the  effect  of  this  microscope  ;but  for  this 
purpose,  we  must  close  the  shutters,  and  admit  only 
a  small  portion  of  light,  through  the  hole  in  the  window- 
shutter,  which  we  used  for  the  camera  obscura.  We 
shall  now  place  the  object  A  B,  (plate  XXI II.  fig.  1.) 
which  is  a  small  insect,  before  the  lens  C  D,  and  nearly 
at  its  focus  ;  the  image  E  F,  will  then  be  represented 
on  the  opposite  wall  in  the  same  manner  ah  the  land- 
scape was  in  the  camera  obscura;  with  this  difference, 
that  it  will  be  magnified,  instead  of  being  diminished. 
I  shall  leave  you  to  account  for  this,  by  examining  the 
figure. 

Emily,  I  see  it  at  once.  The  image  E  F  is  magni- 
fied, because  it  is  farther  from  the  lens,  than  the  object 
AB;  while. the  representation  of  the  landscape  was 
diminished,  because  it  was  nearer  the  lens,  than  the 
landscape  was.  A  lens,  then,  answers  the  purpose 
equally  well,  either  for  magnifying  or  diminishing 
objects.^ 

Mi^s,  B,  Yes  :  if  you  wish  to  magnify  the  image, 
you  place  the  object  near  the  focus  of  the  lens;  if  you 
wish  to  produce  a  diminished  image,  you  place  the  t)b- 
ject  at  a  distance  from  the  lens,  in  order  that  the  image 
may  be  formed  in,  or  near  the  focus. 

Caroline,  The  magnifying  power  of  this  microscope, 
is  prodigious ;  but  the  indistinctness  of  the  image  for 
want  of  light,  is  a  great  imperfection.  Would  it  not 
be  clearer,  if  the  opening  in  the  shutter  were  enlarged, 
so  as  to  admit  more  lio;Iit. 


/'-ui  .   /  . 


Tub.  7jy  J.Y.Huii.phr.iys T'hSL.A-: 


%^    ■*!- 


■f'i 


OPTICS.  303 

Mrs,  B,  If  the  whole  of  the  light  admitted  does  not 
fall  upon  the  object,  the  effect  will  only  be  to  make  the 
room  lighter,  and  the  image  consequently  less  distinct. 
Emily.  But  could  you  not  by  means  of  another 
lens  bring  a  large  pencil  of  rays  to  a  focus  on  the  ob- 
j  ect,  and  thus  concentrate  the  whole  of  the  light  admit- 
ted upon  it ; 

Mrs,  B.  Very  well.  We  shall  enlarge  the  open- 
ing, and  place  the  lens  X  Y  (fi^.  2.)  in  it,  to  converge 
the  rays  to  a  focus  on  the  object  A  B.  There  is  but 
one  thing  more  wanting  to  complete  the  solar  micros- 
cope, which  I  shall  leave  to  Caroline's  sagacity  to  dis- 
cover. 

Caroline.  Our  microscope  has  a  small  mirror  at- 
tached to  it,  upon  a  moveable  joint,  which  can  be  so  ad- 
justed as  to  receive  the  sun's  rays,  and  reflect  them  up- 
on the  object;  if  a  similar  mirror  were  placed  to  re- 
flect light  upon  the  lens,  would  it  not  be  a  means  of 
illuminating  the  object  more  perfectly. 

Mrs.  B.  You  are  quite  right.  P  Q  (fig.  2.)  is  a  small 
mirror  placed  on  the  outside  of  the  window  shutter, 
which  receives  the  incident  rays  S  S,  and  reflects  theia 
on  the  lens  X  Y.  Now  that  we  have  completed  the  ap- 
paratus let  us  examine  the  mites  on  this  piece  of  cheese, 
which  I  place  near  the  focus  of  the  lens. 

Caroline.  Oh,  how  much  more  distinct  the  image 
now  is,  and  how  wonderfully  magnified ;  the  mites 
on  the  cheese  look  like  a  drove  of  pigs  scrambling  over 
rocks. 

Emily.  I  never  saw  any  thing  so  curious.  Now,  an 
immense  piece  of  cheese  has  fallen  :  one  would  imagine 
it  an  earthquake :  some  of  the  poor  mites  must  have  been 


3)04  OPTICS. 

crushed ;  how  fast  they  run, — They  absolutely  seem  to 
gallop. 

But  this  microscope  can  be  used  only  for  transparent 
objects  ;  as  the  light  must  pass  through  them  to  form  the 
image  on  the  wall  ? 

Mrs,  B.  Very  minute  objects,  such  as  are  viewed 
in  a  microscope,  are  generally  transparent ;  but  when 
opaque  objects  are  to  be  exhibited,  a  mirror  M  N  (fig.  S.) 
is  used  to  reflect  the  light  on  the  side  of  the  object  next 
the  wall :  the  image  is  then  formed  by  light  reflected 
from  the  object,  instead  of  being  transmitted  through  it. 

Emily,  Pray  is  not  a  magic  lanthorn  constructed  on 
the  same  principles  ?  ' 

Mrs,  B.  Yes;  with  this  difference,  that  the  light  is 
supplied  by  a  lamp,  instead  of  the  sun. 

The  microscope  is  an  excellent  invention,  to  enable 
us  to  see  and  distinguish  objects,  which  are  too  small  to 
be  visible  to  the  naked  eye.  But  there  are  objects 
which,  though  not  really  small,  appear  so  to  us,  from 
their  distance;  to  these  we  rannot  apply  the  same  re- 
medy ;  for  when  a  house  is  so  far  distant,  as  to  be 
seen  under  the,  same  angle  as  a  mite  which  is  close 
to  us,  the  effect  produced  on  the  retina  is  the  same  : 
the  angle  it  subtends  is  not  large  enough  for  it  to  form 
a  distinct  image  on  the  retina. 

Emily*  Since  it  is  impossible,  in  this  case,  to  ap- 
proach the  object  to  the  eye,  cannot  we  by  means  of  a 
lens  bring  an  image  of  it  nearer  to  us  ? 

Mrs.  B,  Yes ;  but  then  the  object  being  very  dis- 
tant from  the  focus  of  the  lens,  the  image  would  be  too 
sniall  to  be  visible  to  the  naked  eye. 

Mmily.    Then,  why  not  look  at  the  image  through 


OPTICS.  305 

another  lens,  which  will  act  as  a  microscope,  enable  us 
to  bring  the  image  close  to  the  eye,  and  thus  render  it 
visible  ? 

Mrs.  B,  Very  well,  Emily;  I  congratulate  you  on 
having  invented  a  telescope.  In  figure  4,  the  lens  C 
D,  forms  an  image  E  F,  of  the  object  A  B ;  and  the  lens 
X  Y  serves  the  purpose  of  magnifying  that  image  ;  and 
this  is  all  that  is  required  in  a  common  refracting  tel- 
escope. 

Emily.  But  in  fig.  4,  the  image  is  not  inverted  oa 
the  retina,  as  objects  usually  are  :  it  should  therefore 
appear  to  us  inverted  ;  and  that  is  not  the  case  in  the 
telescopes  I  have  looked  through. 

Mrs.  B,  When  it  is  necessary  to  represent  the 
image  erect,  two  other  lenses  are  required ;  by  which 
means  a  second  image  is  formed,  the  reverse  of  the  first 
and  consequently  upright.  These  additional  glasses 
are  used  to  view  terrestrial  objects  ;  for  no  inconveni- 
ence arises  from  seeing  the  celestial  bodies  inverted. 

Emily,  The  difference  between  a  microscope  and 
a  telescope  seem  to  be  this  : — a  microscope  produces  a 
magnified  image,  because  the  object  is  nearest  the  lens; 
and  a  telescope  produces  a  diminished  image,  because 
the  object  is  furthest  from  the  lens. 

Mrs,  B,  Your  observation  applies  only  to  the  lens 
C  D,  or  object  glass,  which  serves  to  bring  an  image 
of  the  object  nearer  the  eye  ;  for  the  lens  X  Y,  or  eye- 
glass is,  in  fact,  a  microscope,  as  its  purpose  is  to 
magnify  the  image. 

When  a  very  great  magnifying  power  is    required, 
telescopes  are  constructed  with  concave  mirrors,  instead 
©flenses.     Concave  mirrors,  you  know,  produce  by  re- 
c  c  2 


306  OPTICS. 

flection,  an  effect  similar  to  that  of  convex  lenses  by  re- 
fraction. In  reflecting  telescopes,  therefore,  mirrors 
are  used  in  order  to  bring  the  image  nearer  the  eye ; 
and  a  lens  or  eye-glass  the  same  as  in  the  refracting 
telescope  to  magnify  the  image. 

The  advantage  of  the  reflecting  telescope  is,  that 
mirrors  whose  focus  is  six  feet  will  magnify  as  much  as 
lenses  of  a  hundred  feet. 

Caroline.  But  I  thought  it  was  the  eyeglass  only 
which  magnified  the  image;  and  that  the  other  lens 
served, to  bring  a  diminished  imaj>;e  nearer  to  the  eye. 

Mrs.  B.  The  image  is  diminished  in  comparison  to 
the  object,  it  is  true;  but  it  is  magnifie«l  if  you  compare 
it  to  the  dimensions  of  which  it  wouhl  appear  without 
the  intervention  of  any  optical  instrument  ;  and  this 
magnifying  power  is  greater  in  reflecting  than  in  re- 
fracting telescopes. 

We  must  now  bring  our  observations  to  a  conclusion 
for  I  have  communicated  to  you  the  whoie  of  my  very 
limited  stock  of  knowledge  of  Natural  Philosophy.  If 
it  will  enable  you  to  make  further  progress  in  that 
science,  my  wishes  will  be  satisfied  ;  but  remember 
that,  in  order  that  the  study  of  nature  may  be  productive 
of  happiness,  it  must  lead  to  an  entire  confidence  in 
the  wisdom  and  goodness  of  its  bounteous  Author. 


INDEX. 


Air,   16, 23, 41, 75,  205,  247, 

274. 
Air-pump,  46,  208. 
Ang-le,  66. 

acute,  67, 
obtuse,  67. 
of  incidence,   68,   242, 

262. 
of  reflection,  68,   231, 

243,  262. 
of  vision,  254,  262, 
Aphelion,  114. 
Arctic  circle,  139,  150. 
Atmosphere,  155,  194,  205, 
220,  246. 
reflection  of,  224. 
colour  of,  278 
refraction  of,    271, 
276. 
Attraction,  15,22,35,272. 

of  cohesion,  22,  50. 

178,  206. 
of  gravitation,    27, 
47,  107, 122,  145, 
171,  205. 
Avenue,  256, 


Auditory  Nerve,  232. 
Axis,  118. 

of  motion,  72   82. 

of  the  earth,  139,  1488 

of  mirrors,  265. 

of  a  lens,  278. 


B. 

Balloon,  45, 
Barometer,  211. 
Bass,  233. 
Bladder,  208. 
Bodies,  15. 

elastic,  60,  74. 

luminous,  237. 

sonorous,  223. 

fall  of,  35,  39,  45,  5S. 

opaque,  238,  272. 

transparent,  238,  272. 
Bulk,  24. 


Camera  obscura,  248, 287, 302. 


308 


INDEX. 


Capillary  tubes,  27. 
Centre,  72. 

6i  gravity,  72,  76,  80, 
82,  172. 

of  motion,  72,  82,  173. 

of  magnitude,  72,  79. 
Centrifugal  force,  74,  111,  143, 

172. 
Centripetal  force,  74,  111. 
Ceres,  126. 
Circle,  65,  141,  144. 
Circular  motion,  71,  IIL 
Clouds,  194. 
Colours,  34,  279. 
Comets,  128. 
Compression,  63. 
Concord,  233. 
Constellation,  128. 
Convergent  rays,  264,  267. 
Crystals,  18. 
Cylinder,  78. 


Bay,  118,  157. 

Degrees,  66,  141,  149,  258. 

of  latitude,  142,  167. 

of  longitude,    142, 
167. 
Density,  23. 
Diagonal,  70. 
Diameter,  141. 
Diurnal  118. 
Discords,  232. 
Divergent  rays,  264 
Divisibility,  15,  18^. 


E. 

Earth,  28,  107, 126,  133,  137. 
Echo,  231. 

Eclipse,  165,  170,  241. 
Ecliptic,  129,  139. 
Elastic  bodies,  60,  62. 

fluids,  23, 42, 178,  205. 
Ellipsis,  113. 
Essential  properties,  15 
Exhalations,  19 
Extension,  15, 17 
Equator,  139 
Equinox,  150,  152 

precession  of,  157 
Eye,  247 


F. 

Fall  of  bodies,  35,  39,  45,  55 
Figure,  15,  17 
Fluids,  178 

elastic,  178,  205 

equilibrium  of,  179,  21€ 

pressure  of;  1^1, 198, 212 
Flying,  60 
Focus,  266 

of  convex  miiTors,  266 

of  concave,  268 

of  a  lens,  278 
Force,  50 

centrifugal,  74,  111,  143, 
172 

centripetal,  74,  111 

of  projection,  75,  109 

•f  gravity,  27,  107,  121, 
156,  205 


INDEX. 


309 


Fountains,  203 
Friction,  103,  203 
Friii^id  zone,  140,  149 
Fulcrum,  82 


General  properties  of  bodies, 

15  * 

Georg-ium  Sidus,  127 
^lass,  276 

refraction  of,  276 
burning",  283 
Gold,  187 

Gravity,  27,  35,  47,  50,  55,  75, 
76 


H. 


Harmony,  233  • 
Heat,  25,  155 
Hemisphere,  139,  150 
Hydrometer,  191 
Hydrostatics,  178 


Image  on  the  retina,  249,  259, 

Imag-e  reversed,  251 

in  plain  mirror,  260 
in  convex  ditto,  264 
in  concave,  264 

Impenetrability,  15 

Inclined  plane,  81,  100 

Inertia,  15,  21,  50 


Juno,  126 
Jupiter,  126,  170 


Lake,  200 

Laiiliide,  142,  167 

Lens,  2r7 

convex,  277 
concave,  278 

Lever,  81 

first  order,  87 
second  ditto,  89 
third  ditto,  90 

Light,  238 

pencil  of,  239 
reflected,  242 
of  the  moon,  245 
refraction,  of,  271 
absorption  of,  283 

Liquids,  178 

Longitude,  142,  167 

Luminous  bodies,  237 

Lunar  month,  164 
eclipse,  165 

M. 

Machine,  81,  99,  103 
Magic  lanthorn,  305 
Mars,  126 
Matter,  15,  58 
Mechanics,  81 
Mediums,  238,  272,  280 
Melody,  234 

Mercury  planet,  124,  171 
Mercury,  or  quicksilver,  212 
Meridians,  141 


310 


IJSIBEX. 


Microscope,  294 

single,  500 
double,  302 
solar,  302 
Minerals,  18 
Minutes,  141 
Monsoons,  223 
Month,  lunar,  164 
Momentum,  57 ^  86 
Moon,  120,  121,  127,  163,  171 
Moon-lig-ht,  245 
Motion,  21,  49,  58,  60, 

uniform,  52 

perpetual,  53 

retarded,  54 

accelerated,  54 

reflected,  ^S 

compound,  69 

circular,  71,  111 

axis  of,  72,  82 

centre  of,  72,  82  172 

diurnal,  118 
Musical  iiTstruments,  233 
Mirrors,  260 

reflection  of,  260 

plane  or  flat,  264 

convex,  264, 

concave,  264,  267 

axis  of,  265 

burning",  268 


N. 

Neap  tides,  174 
Nerves,  249 

auditory,  232,  249 


Nerves,  optic,  247,  249 

olfactory,  250 
Night,  118 
Nodes,  149,  150,  159 


Octave,  233, 234 

Odour,  19 

Opaque -bodies,  238,  239 

Optics,  237 

Orbit,  124. 


Pallas,  126 
Parabola,  7^ 
Parallel  lines,  38 
Pellucid  bodies,  238 
Pencil  of  rays,  239 
Pendulum,  146 
Perihelion,  114 

Perpendicular  lines,  38, 65, 153 
Phases,  164 
Piston,  215 
Plane,  139 

Planets,  116,121,  156 
Poles,  139 
Polar  star,  150, 168 
Porosity,  63 
Powers,  mechanical,  81 
Projection,  75^  108 
Precession  of  the   equinoxes 
157 


INDEX. 


311 


Pulley,  81,  93. 
Pump,  46,  47. 

sucking  or  lifting",  215. 

forcing,  217,  219. 
Pupil  of  the  eye,  247 


Rivers,  193 
Rivulets,  197 


S, 


R. 

Rain,  194. 
Rainbow,  282. 
Rarity,  24 
Ray  of  light,  238 

of  reflection,  242 
of  incidence,  242 
Rays,  intersecting,  248, 
Reaction,  59 
Receiver,  46 
Reflection  of  light,  242 

angle  of,  68,  262 
of  mirrors,  260 
of  plain  mirrors,  264 
of  convex  mirrors, 

264 
of  concave  mirrors, 
264 
Reflected  motion,  65 
Refraction,  271 

of  the  atmosphere, 

274. 
of  glass,  276 
of  a  lens,  277 
of  a  prism,  278 
Resistance,  82 
Retina,  247 

image  on,  2iO 


Satellites,  121,  167, 170 

Sj.turn,  127 

Scales  or  balance,  82 

Screw,  81,  101 

Shadow,  165,  240 

Sidorial  time,  158 

Sigl  t,  249 

Signs,  Zodiac,  129,  139,  142 

Smoke,  20,  43 

Solar  microscope.  302 

Solstice,  149,  150 

Sound,  226 

acute,  233 

musical,  232 
Space,  51 
.  Specific  gravity,  185 

of  air,  211 
Spectrum.  280 
Speaking  trumpet,  231 
Sphere,  39,  78,  144 
Springs,  193 
Spring  tides,  174 
Square,  122,  127 
Stars,  117,  128,  158,  168 
Storms,  220 
Substance,  15 
Summer,  114,  149 
Sun,  107,  122,  238,  275 
Swimming,  61 
Syphon,  199 


312 


INDEX. 


T. 

Tangent,  74,  111 
Telescope,  30^5 

reflecting,  305 
refracting",  305 
Temperate  zone,  140, 151 
Thermometer,  214 
^rides,  171 

neap,  174 

spring-,  174 

aerial,  225. 
I'ime,  157,  160 

siderial,  158 

equal,    .61 

solar,  161 
Tone,  233 

Torrid  zone,  140,  151,  220 
Transparent  bodies,  238 
Treble  and  bass,  233 
Tropics,  lo9 


Undulations,  229 
Unison,  234 

W. 

Waters,  178,  196 

spring-,  196 
rain,  196 

level  of,  180, 187,  194 
Wedge,  81,  lOa 
Weight,  24,  34,  144,  185,  207» 

2U8. 
Wheel  and  axle,  81,98 
Wind,  220 

trade,  221 
periodical,  223,  230 
Winch,  lul 
Winter,  115,  150 


V. 

Valve,  216 

Vapour,  25,  43,  194 

Velocity,  51,  85 

Venus,  125,  171 

Vesta,  126 

Vibration,  229 

Vision,  254    ^ 

angle  of,  254 
double,  259 


Year,  157 

siderial,  159 
solar,  160 

Z. 

Zodiac,  129 

Zone,  140 

torrid,  140, 151,  220, 275 
temperate,  140,  151 
frigid,  140,149 


THE    END. 


)^3 


-^^TT-WH^^^II^KSI^— ; 


